Hi Bunny Girl
That just sounds horrific.
Any chance you can ask your original vet (the one who referred you) for help? Or even ask the zoo for advice? It reads like he got a bite from a white tailed spider with dirty fangs and now it's all infected. Even a human doctor who specialises in skin infections might be able to help with advice.
I'm thinking one of those stretchy bras might be the go. Or pantihose with a tail hole cut in... Even if you have to pad the convexity with socks or something. Clingwrap? It's hard to think of anything that will stay put in that spot. There's a sort of tubular bandage or knee strapping with Velcro that might do it. You can buy rolls of the tubular stuff and then maybe wrap it in a loop and X formation? Or a long bit of t-shirt material from a spotlight or something. Ie stretchy but not too stretchy.