Oooh, no bathtub option, shower only? That is a puzzler. No, there's no good way to isolate your foot because almost any skin/plastic/tape seal on your leg/calf/knee is going to be imperfect. So water will get in. I think you're stuck with sponge baths, unless you can source out a tub in the building that you can borrow. Might want to check with friends, too, and see who has a nice tub.