I think I ended up with plywood for the base. It's really heavy. I hope it lasts, but if it doesn't, it will be a simple matter to transfer it all to a new base.

Sorry, I didn't explain the upside down bit very well. I meant, I can't turn it upside down very easily because of all the stuff on the handlebar. (It's a trekking bar, which is a kind of weird looking thing.) So I try to hold it up and turn the pedals backward--but the whole procedure was precarious and I kept knocking into things and it kept wanting to fall on me or away from me...I was holding it up with my head so that my right hand could turn the pedal and my left hand could hold the rag onto the chain.

I needed two more hands to make that one work. Or more coordination. Balance is NOT my strong suit. Amazing I manage so well on a bike...