Michelle @ CRAFT points us to Leo Kempf, who writes:
I made this house for my cat, Olive. She enters through a door in the bottom side and then ascends a ramp, which boosts her to the upper level. The front wall is plexi-glass, the floor is 2.5 inch thick old sheepskin rug, and my wife made some small paintings that hang on the walls. There is also a big cardboard scratching porch. Cats love to scratch and relax on cardboard. The top is removable and the glass slides out for maintenance.