I've been in your shoes a few times. Can you put the hampers behind a closed door?
Just sit down and think about what you really want to do for these cats and yourself, and then think about what you really can accomplish given your current situation, and then make a decision from there.
Just make sure, when/if you return them to the shelter, that you explain everything, you don't want it to stress your other cats out, you don't have a lot of money to take on these foster cats right now and wish to see them to another caretaker.
I've seen it all too many times when a rescuer drops an animal off saying "yeah they started peeing every where." And gave no further explaination, there after the reputation plunges. Thusly you'll never be able to foster through them, or anyone they know again.
Some people can/will look down upon you for returning these fosters to a shelter over a "small litter box issue". But me, and others like me, understand your situation.