Here are a few pictures of the tree my hubby built. The middle piece is a landscape timber I am pretty sure. The shelves are just scrap wood. Some pieces are wrapped with twine and some are covered with nailed on leftover carpet. The base is a very heavy piece - like a half of a coffee table size and then the braces you can see on the last picture - not the prettiest but the tree is so high it needed lots of support. It was all scraps except the middle he bought at the lumberyard but still only about $10 for that. Of course he always has lots of scrapwood around and we had carpet because we had the house done this spring. He looked online at cat trees and just sorta designed it from some that he saw and liked.
He is also a furniture builder and carpenter but said this project was easy enough for a beginner.