Feet nibbling is pretty serious. No way in hell can that be a courting method. I would never go for a male turtle that nibbles on my feet, personally.
I was in the same situation you were- I have a bigger male that harasses the smaller female. They were first in a 10 gallon and I discovered the male stalking/biting the females neck. I thought getting a bigger tank would work and moved them into a 50-55 gal. 2 weeks later, little turtle gets an eye injury
I know the room situation sucks. But seriously, either move them into a bigger tank, get a sheet of egg crate or some other divider, or separate them for good. Maybe put the big one in 55 and little one in 20? Though 20 gallon is really cramped, they'll both be happier and have a better life without facing each other 24/7.
As other mentioned, you can get plastic/storage containers. There are 55 gallon containers for less than $20.
Good luck!