When I broke my arm as a kid, to shower I used plastic garbage bags and tried not to get it wet as best that I could.
Unfortunately I don't know of a better way than that.
Sorry, you're going through this. I've been through it before and it definitely sucks.
There are things available that cost a bunch and really don't do much better than what @dfoulk said. Sorry, but that's what I did for my patients when I worked rehab nursing.
Get some surgical tape and don't skimp on the garbage bag quality.