I'd like to bring the Bike With 2 Brains back to Burning Man this year so I've been thinking about replacing the back wheels with ones that wouldn't break so easily. My dad gave me a couple 9" heavy-duty equipment casters so I figured I'd use them. I had originally thought to use larger ones, but free wins.
I tinkered around with the copper model a little to see what would look least stupid — 24" front wheels and 9" back wheels will probably look pretty silly. I decided to maintain the existing frame design as best I could (i.e. least amount of work to change) and just extend a leg with a decorative (and suspension-flex) bend in it and weld it to a plate, bolted to the caster. I had the plates from straight wheels I bought for my bicycle trailer, so I canabalized them for the project. I bent a piece of 1" water pipe then cut it so the bottom was perpendicular to the straight part of the pipe on top which gets welded to the old steering tube used with the original back wheels. It looks like the back end will ride a little higher with the new wheels, but not by much.