A posture arch is a bit of bent wood you lie on to help relieve back pain. I use one at the gym I go to (it’s attached to a physiotherapist so there’s lots of odd things lying around). The only problem is they charge $120 for this piece of bent wood.
Here’s the company that makes them so you can get an idea.
Being a bit cheap I thought I could make one. So I traced the outline of one onto a piece of paper and used some scraps of wood I had lying around to come up with this :
It’s not quite as elegant as the original but it’s pretty similar. I’ve been using it a few times a week for a month now and it seems to work as well as the posh one.