He Chose a Shelf
This bookshelf sits between my two desks. I had an empty shelf I hadn't placed anything on yet, and the cat decided he would make it his own. After enough times climbing up there, I put some old fleece pajama pants down for him, and he's been in love. It's not an official cat condo, but it's another cozy space that he loves to curl up in.
He loves nooks and crannies, so while I saw an empty shelf that some books might go in, he claimed it as his own. I love looking over and seeing him sleeping there while I'm working on projects. He always looks so comfortable!