If you have any pet crates, you can take the doors off of them to create bed areas for the cats. I have one outside for the neighborhood cat that visits. I put a pet bed in it and he can come and go as he pleases. If you have wind and rain, you will need to figure out how to protect the bed from getting wet but otherwise it works well.

For an inexpensive really large litter box, take a plastic food grade (cooking oil type) 50 gallon drum and cut off the bottom 12". My spouse even carved handles out for carrying (for this, cut at 16"). I will take a picture and post as soon as I can. You will also need to figure out how to cover this as well. (I only bring this up as the cats will be limited in space in the enclosure and the number of cats will add up the "poop" etc. in a hurry.)

Smile, it will make people wonder what you are up to.Clap