When I was in the Army many years ago, I was an assistant instructor in a rappelling school. We didn't have any fancy harnesses, so I taught each trainee how to tie a Swiss seat using a length of nylon webbing. One inch wide is the minimum, but you might want 1.5 inch for better comfort. After it is tied, you need somebody to inspect it for safety. I can't think of what would have been better, because that is all we had. One carabiner was necessary to hook onto the standing rope, although two can be used (in opposition) for safety. Back in those days, all of the rope was half-inch green laid nylon rope (not kernmantle) similar to Goldline.
I also had to demonstrate the body rappell, and it uses no harness at all. I still have the burn scars to prove it.