Of course, in my brilliance, I forgot that I didn't have an office chair that I could cannibalize.
So first I had to find one. I went to a local used business furniture store and asked if they had any damaged chairs. The best ones have a square base to attach to the bottom cushion. The one I wound up getting was fairly close to that ideal.
While I was there, I bought another chair cheap and got a free loveseat to boot. A huge coup for someone who otherwise has trouble talking to people.
Admittedly, it helps to live in a large metro area where lots of businesses need and unload furniture. But you may be able to use a castoff from friends or family whose cushions have flattened out or become otherwise unusable.
You might just now think to yourself, "Dude, with all that junque in your garage, why do you NEED a rolling chair?" But as an instructabilista, you already know the answer...which may go something like, "If I only worried about what I need, I'd live in a cardboard box with an inflatable doll (from American Yakuza, an under-appreciated film with Viggo Mortensen.)"
Step 1: The Mating Ritual -- Choose Your Material
A perfect match between the chair base and the racing seat has absolutely no chance of occurring, or if it did, would have wasted a piece of luck better spent on winning a lottery.
So assume you'll need to make some kind of adapter.
What to use? The average Joe or Jane has three choices --
1) Wood -- easy to cut and drill but you'll need at least 3/8" (10mm) thick plywood or maybe 1x2 boards of something not too soft. Keep in mind the grain of the wood in relation to your adapter design.
2) Aluminum (Aluminium) -- almost as easy to cut and drill as wood but you'll likely need 1/4" (6.5mm) thickness minimum.
3) STEEL -- takes a bit more work to cut and drill but even 1/8" (3mm) thickness will work in most cases. 3/16" thick steel will handle just about anything.
Note that most car frames are made with 1/4" (6mm) mild steel, and they support static loads of over 2000 lbs (900 kg) no problemo.
Other materials -- plastics, composites, or exotic metals such as titanium -- if you have these, you probably don't need to read this instructable.