Every piece I have made has been from a design I saw on the net with my own special twist to it. Most basic funniture/gear you will see is very simple to build. Copy pictures and draw it up on paper as a print, then make your own modifications to it there. Check everything for measurements that will suit your needs too. Heights have generally been an issue for me since I'm 6'3, so tables, benches, vault horse, etc have to be a bit taller for acess. (I refuse to squat lol) Take into account your sub's height/weight as well as your own. Nothing's worse than having to bend yourself in half to have acess for any sex with a piece you made too short, been there, done that!
My Y-chair is a perfect example. I got a few pieces of 6x6 from a jobsite dumpster and designed it around them as the main frame. Since my wife/sub is a bigger girl, I modified the legs to be a bit wider than most and added a bit of a seat, rather than just the area where the upper-leg pieces intersect.. I made the top of the "cross" taller than those I have seen, and added a bent-pipe frame with a pulley for breast play. All modifications to the origional design I saw have worked perfectly. It weighs a ton, but once in place, was screwed directly to the floor and hasn't moved in 6 years.