As luck would have it, the commercial CanDoo system is NOT the only way to relocate your spare tire under the car. I did the mod myself for a total of about $85. Of course, I did it with the help of Dan Smith of Fresno CA. Dan and some Miata nuts on the left coast figured out how to do this and posted a lot of details and pictures on the Miata.net.
I just downloaded all the info and printed it out, pictures and all. I picked up all the necessary tire remounting pieces at the local Handyman hardware store (6" x 1/2" bolt, matching nyloc nut and wingnut, two 3/16" thick 2" diameter washers, one thin 2" diameter SST washer, and a hitch pin/spring clip). Per a good suggestion from Jack McCombs, I drilled thru the diameter of the bolt about 1/2" from the end to slip the spring clip thru the bolt to ensure the wing nut didn't accidentally come off. Hardware costs: $5.
I used the left hand drain plug hole in the floor of the trunk to attach the tire hardware (no drilling). Remove the rubber plug, slide a thick aluminum washer over the bolt and insert it thru the hole from inside the trunk. From underneath the car, slide on the second aluminum washer and secure the bolt with the nyloc nut (takes two people, one in the trunk and one under the car). Seal the junction of the trunk floor and the bolt with Silicone RTV on both sides.
The tire attaches thru one of its lug holes (NOT the large center hole). The tire will be offset towards the front of the care. Another good idea from Jack McCombs is to invert the tire/wheel assembly and position it so that the inflation stem is pointing downward for easy access without having to remove the tire from the car. Secure the tire with the SST washer, wing nut, and safety clip. You may have to bend one of the stock exhaust hangers up out of the way for the forward part of the tire to lay flat up against the floor of the trunk. I even cut/bent the stock heat shield to retain its use in deflecting some muffler heat.
The muffler you need is a Walker DynaMax Mini-Turbo Flow, Part Number 17704. It has a center inlet and an offset outlet. I purchased mine for $18.75 from Main Auto Parts in Fairborn (Ohio) I next took the car to a local muffler shop that was willing to do REAL custom work. I checked out about 6 local shops and some wouldn't touch this kind of work. See Mike at Muffler Brothers on Maple Avenue in Fairborn-- 513-879-4431. (Mike has now done this mod to 6-7 Miatas and is very good at making each one look like a factory job.)
I showed the technician all of Dan Smith's pictures and directions and the guy went to work. He cut off the intermediate pipe at the differential, bent up a connector pipe to the muffler and another one from the muffler to the tip. All pipes from the cat back are 2"D. All joints are welded, no clamps. He also custom welded new hangers in the exact locations needed. Pipes and labor: $43. I also purchased a SST exhaust tip from him for an extra $17.
Everything clears well, tip exactly centered in the bumper cutout, no heat problems with the spare tire or trunk contents. Since I already had a Walker aftermarket TurboFlow DynaMax system, my intermediate pipe has a welded-in resonator. This in combination with the new Mini-Turbo muffler sounds just as good as the system before -- deep and throaty, not raspy.
I finished off the installation with a few strategically placed sprays of Hi-Temp flat black engine paint on parts of the muffler and pipes to visually hide any shiny spots. From the rear of the car, you cannot see any of the spare tire, and only about an inch of the lower part of the muffler. The inlet pipe to the muffler comes up across the rear of the diff at an angle. The flat black hi-temp paint makes it virtually invisible now.