Got a 7ft thruster that I want to use for kitesurfing. Figured I'd add straps to assist in launch. What's a good position for the back strap?

It should be along the centerline of the board in the same place as your foot would be while regular surfing. Check out your back foot placement while surfing. I'd also add a layer of glass over the top "foot strap" area of the board.
I've done this to several surfboards and all the boards ended up trashed due to cracks where your feet are and along the rails near your feet. But they are fun in the waves.

Scott