I'd get something like the butane bottle and flush the air out after transferring (watch for ignition sources!), you will probably have good results. Glass is superior to most plastics as far as permeability, make sure the lid is a non corrodeable material.
Melanie, looks like you have it in good control, watch that duck tape though, the feathers tend to stick to the wet film.
I've gotten as many as 6 rolls of 35mm through one liter of C-41 extending the times per the directions on the kit, but the Flexicolor kits I got last doesn't have that option in the directions. I suspect you can do the same there but Kodak won't make any promises on the quality of the results.