Everyone always likes this idea when they see it, and I haven’t seen anything like it yet on Ikea Hacker. It cost maybe $45? The shelf was in the as-is section in Ikea for $20-25 because it has plastic stuck between the shelves that got caught when they nailed them together. I yanked/cut most of it away. The sisal rope I think was $10 from the hardware store, and no glue or staples needed to attach it. $10 Petco donut bed sits on the top shelf. Sweater and two random carpet samples on the other shelves were free. Not the loveliest, but easy, fast, and not really any uglier than those carpet trees that run $100-$200.
Although you could do this with a standard shelf, the corner one makes it super easy to stagger the shelves so the cats have easy steps (especially nice for my older cats).