Basically, build them a chicken coop with a tall run and plenty of shelves to sit on. My aunt has one that she calls catlandia. She used to live in a neighborhood where someone was poisoning cats. Plus there were injuries from fights and accidents with cars. Before catlandia, she had a very simple rectangular one that she gave me when she moved. We slapped wheels on it, installed roosts, and now it's our tractor/grow out pen.
Agreed, like a chicken coop. Predator proof, shade, wind block, sleeping space, and perches, different heights with different views. We'd love to see pics when you are done! Also things they can scratch on. My cat goes straight to the firewood pile and scratche the bark when I let him out, so maybe a tree trunk/stump or branch.