I broke the derailleur hanger from a Trek 2200 after many years of hard use. This happened to be a terminal condition for this bike. Couldn't bring myself to toss it after so many wonderful miles - so it sat around my garage. Used the top tube and down tube for some other project a few years ago, which left the rear triangle lonely in the corner. I decided the triangle would make a decent shop stool, just had to make a few pieces and reclaim a seat tube from another spare bike.
Firs step cut out the rear triangle. This bike already had the top tube and down tube salvaged, I just needed to make a cut closer to the tubes. This happened to be a carbon tube bike, with aluminum lugs.
Making a trim cut in the carbon/aluminum is a piece of cake with a saws-all.
A closeup of the rear end, I cut off the derailleur hanger to make the back level. This proved to be too unstable - needed to have a wider base. So I built a wider base that fit where the hub used to live. The brace is built from a couple pieces of scrap wood (walnut). I drilled a hole through the scrap that fits between the rear triangle and inserted an old quick release skewer to hold it all together.
Next step - making something to sit on. I happened to have an extra seatpost laying around. This one is a Dura Ace 25th anniversary edition - way too nice for a shop stool but the only one not in active use - oh well. Took a piece of 1 inch white oak and drew a rough outline of a bike seat and cut on the bandsaw. Took the seatpost assembly apart and only used the post and bottom adjustment bracket - screwing from the bottom into the seat. Just insert the seatpost and set to your preferred height.