Hey Lou! I saw one recently done by Lazy Guy DIY that I really thought was cool! https://lazyguydiy.com/ultimate-diy-cat-condo/
40 ideas: https://bigdiyideas.com/40-cool-diy-cat-tree-kitty-condos-cat-climbers/
You would build it with plywood and 2 by 4 s then cover with carpet you could eather glue the carpet or staple it on
27 different plans!