I agree with Valo. I admire your commitment to these cats. They will be fine with the acommodations, and it's by far a better option than running free and being hit by a car or attacked by a dog.
I would not use a plastic litterbox, unless there is no chance of it getting rained on. I would create a "sandbox" iin the ground for them. You can scoop the poop and disinfect the sand ever so often by spraying with bleach water and rinsing with garden hose.
Once you add different perches, maybe a large limb for them to climb and toys, they will be fine.

Caroline
~I don't suffer from madness. I enjoy every minute of it.~