Cam buckles, I think.
I did some web surfing a few months back trying to figure out how much it would cost to make your own ring straps from online webbing retailers. I came to the conclusion that unless you're going to be making multiple pairs, you're not saving much money. Rogue Fitness sells ring straps separately for something like $25, which I believe is only marginally more than the cost for the materials and won't require your own labor.
However, if you're doing this for the love of DIY, keep us posted on the project.