Miata Rear Shelf Extension

Doug Thomson
Fine Print: Please note that while the pictorials "appear" to be step-by-step, there may be important information not explicitly stated.
Check your Mazda manual. Do all mods at your own risk! No guarantees!

Our dog likes to sit on the rear shelf of our Miata. When the top is up, no problem, but when the top is down, there's not much space - Syd is a full figured girl and needs a little extra leg room. This little shelf is simple to make, proves that it is possible to bend Plexiglas or Lexan, looks good, and is easily removable in that it uses Velcro and an existing carpet button to hold it in place. I have patterned this on a 99 Miata. You may have to make adjustments for earlier models.

One day, I'll add a thingie showing how we made provision for a dog seatbelt harness.

Here's What You Will Get When Finished
Click on Thumbnails to See Larger Images

Sydney's red pillow sitting on top of shelf extension.
Here's Syd using extension - note that she won't ride without the red blanket. Windblocker is the other design on this site.

  • A piece of 1/4" Plexiglas or Lexan about 13" x 14"
  • Scraps of 3/4" plywood to make bending form - don't use mdf or similar products ... use plywood.
  • 1/2" high density foam to make the padding (I used a blue backpacking foam ... cheap and easy to find).
  • Fabric to cover your shelf extension.
  • Screws for building bending form
  • Self-adhesive Velcro (get the industrial strength stuff ... it has a rubberised back, serious adhesive, and much stronger hooks and loops. You should be able to get it at a good fabric shop.)
  • Contact cement (don't use the latex stuff, it won't stick well enough. You need the old fashioned dangerous stuff that's very highly flammable and must be used with loads of ventilation ... nasty stuff but provides serious stick.

  • Mitre saw to cut angles for bending form ... could also be table saw, radial arm saw, whatever.
  • Clamps: 2 - 6" & 2 - 12"
  • Drill and bits. (1/4" and 1")
  • Screwdrivers
  • Hammer
  • Jig saw, coping saw, or something similar for cutting out Plexiglas. (Cutting at a slow speed is important as this stuff will melt and fill the curf that you just cut.)
  • Oven ... you will have to convince your spouse that this really won't mess up the oven.
  • Heavy leather gloves, oven mitts, etc.
  • Scissors
  • 1" Sponge brush for spreading Contact Cement.
Method (note: large images have textual explications)    
Images 1 & 2
Here's the pattern for your shelf extension and a shot of the bending form with the 1/4" Plexiglas already bent. Because this shelf extension will be covered, you don't have to worry about the sharp edges of the form creating blemishes on the Plexiglas. Ergo, I didn't bother to round the edges of the form or to cover the form with cloth. You will, however, want to round all of the corners on the plexi. It will help you when attaching the foam padding.

Image 1

Image 2
Look at Image 2 (large version) for additional notes. Make sure that the form is built very solidly, as it will have to withstand considerable clamping force. I suggest clamping the Plexi to the form using the sandwich method. The 2 - 1/2" base and 6" platform will overhang the form when you put it in the oven. As they heat in the oven they will begin to sag over the form (you will finish the rest of the bending).    
Heat your oven to about 300 degrees F. Clamp the plexi to the form - make sure it is very firmly clamped. Put the form in the oven. Keep very careful watch of the Plexiglas in the oven. As the form heats, the base and platform sections of the Plexiglas that are overhanging the form will begin to sag slightly. Prod the Plexiglas slightly with your oven mitt, it should feel rubbery. If you hear a crackling sound get your form out of the oven quickly as the Plexiglas is beginning to break down ... bubbles of gas will be forming in it and will weaken it considerably.

Here's where you will need two sets of hands. You must move quickly as the Plexiglas will cool quickly ...

  • Have your partner get ready with the long clamps.
  • Put on your heavy leather gloves or oven mitts
  • Remove the form from the oven and make sure the clamps on the sandwich are tight.
  • Use the two pieces of plywood shown in Image 2 to bend the overhanging ends of the Plexiglas around the form (gives you support and keeps you from getting burned) ... this will be difficult. You may wish to preheat these boards along with your form and Plexiglas. This will help keep it hot.
  • Have your partner quickly clamp the pieces of plywood, Plexiglas, and form together, creating a tight bend on the corners. As long as you move quickly, and the Plexiglas is still hot, the bends will form nicely and you do not risk breaking the Plexiglas. Speed and pressure are the secrets.

Congratulations, you've done the hard part.

Images 3 & 4
Carefully remove the button that holds down the shelf carpet that sits directly behind the shelf extension. Using a permanent marker, mark the location of the carpet button, and storage compartment lock on the Plexiglas. Drill holes as shown in Image 4. Sand and finish the edges of the Plexiglas.

Image 3

Image 4
I forgot to take a picture of the padding on the top ... sorry. Anyhow, you will use contact cement to attach the foam to the top platform. Lay the foam on top of the extension (platform) and try bending it around the platform to form a nicely rounded edge on the shelf. This high density foam is quite stiff and you will need to wrap around about 2" to have a large enough glue line to hold the foam. You will need to cut the foam about 2 - 1/2" larger than the top platform on 3 sides (passenger, driver and front). Cut the back side flush but with a taper to ease the fabric you will apply later. Glue the foam to the top, roll and glue the front edge, trim excess material, and role and glue the sides.

Images 5 & 6
My wife, Linda, applied the fabric for me, and here I defer to her talents - which are considerable. Here's what I managed to glean from her efforts.

  • Don't glue the fabric to the top of the padding.
  • Don't put fabric on the bottom or back of the Plexiglas. That's where the self-adhesive Velcro attaches.
  • The front sides and all edges must be completely covered ... that takes.
  • Be frugal with the contact cement ...
  • Make cut-outs for the key and carpet button.

Image 5 (this is really black, honest)

Image 6

On the very back of the console (next to the carpet) attach a 4" strip of Velcro loops. Make sure you thoroughly wipe that area of console with alcohol before attaching. Stick a 4" strip of Velcro hooks to the bottom of the Plexiglas (so it hooks to the strip you put on the console ... duh!). Now cover the back of the Plexiglas with Velcro hooks ... they will stick to your Miata's carpet.

Now position your finished shelf extension, press the Velcro in place, and insert the carpet button through the Plexiglas and into its original hole. Your shelf extension is now firmly in place.
More Pictures  

Driver's side view. I'm 6' and extension doesn't interfere with shifting.

Passenger side view.
Poor angle doesn't really tilt downhill.

This image shows the
carpet button recessed in the Plexiglas.

Note how the console
storage opens under the platform.

Syd's casual hanging foot.

Back to the Garage

8 April, 2006