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.)"