Thanks everyone for the ideas!

Citric Acid - I like this idea. Probably a good solution to the toxic issue.

Trash bag idea - This could work and I like it, but I want to be able to check the progress and scrub a bit of the rust off with a soft brush as I go. I'd have to redo the bags each time. Viable option though.

2x4 and plywood idea - I like this too. I have most of this on hand so it would be easy. I think I may go this direction.

Motel bathtub idea - best idea of all. They actually use oxalic acid to remove rust from toilets, sinks and bathtubs in very old bathrooms and kitchens. I tried to convince my wife to let me use ours (we rent an apartment) but it didn't fly. Go figure.