Yep, I have. (Well, that is, SpouseO did most of the work, but I helped!) Because my cat is supremely spoiled, however, I’m not certain we really saved any money.
Our tree is about 5.5 feet tall, and we made it look like a tree - it has a round trunk, covered with brown corduroy fabric, three “branches” that are really platforms that are covered with mottled green fabric, a top platform, again, covered with mottled green fabric, and a low standing mushroom next to the trunk (so she can get to the first branch).
For the platforms, we cut kidney-bean shaped forms out of, well, not quite plywood but something similar - it was about 1/4" thick. The trunk is made out of 2x4s, I think 4 total. The trunk’s attached to two 2’ x 3’ish sheets of 3/4 plywood for stability. SpouseO’s an engineer, so I believe he cut an outline of the trunk in the first plywood sheet, fit the trunk into that, then screwed and bolted it into the bottom sheet (again, for stability. I’d read that cats won’t use something they don’t think is stable, but we probably went overboard on the stability issue. Then again, it’s certainly very, very stable - I’ve had 12-year-olds playing on the stupid thing.)
The branch platforms were attached to the trunk with simple 2x4s cut at angles, then the whole thing was wrapped with foam, stuffing, and fabric. The foam went around the trunk to make it more roundish, sheets of batting went around each platforms’ tops to make them comfy, then the fabric - polartec fuzzy stuff for the platforms’ tops (again, for comfyness - clearly my cat is way spoiled), green fabric stuffed with fiberfill to make the branch platforms’ bottoms look more “treeish” (as well as making a bolster on the top platform for leaning), and the brown stuff for the trunk. All fabric and foam was secured with a staple gun; all wood attachments with screws.
The mushroom is a simple round cut out of the 1/4" stuff, secured to a 6" diameter (or so) heavy cardboard tube (I think) and attached to the base. It’s covered with silvery gray fur - like you’d use for a costume, along with the requisite stuffing and batting and foam. The base is covered with a carpet scrap.
All-in-all, the thing probably cost us $200 (again, because we spoil the cat) and about a weekend’s worth of work. But it’s cool as hell, it really looks like a tree, and for its size, it’s a good deal cheaper than we could’ve spent buying a pre-made one. (We used fabric because we could; our cat’s declawed in front. We got her that way; no flames please. She’s a strictly indoor cat.) And she loves it, so it’s all good. We’ve even thought of making them for fun and selling them on eBay, but shipping for that monster would be a fortune. Did I mention it’s stable?