Sorry for the back to back post, but just thought of something. Here's a good source for the straps. Just picked these up from a local junkyard. I just grabbed the first ones I saw but if you really wanted to, you could look for the great old white GM '80's Monte Carlo! I think they had white seat belts and you could dye them.
I have to say, that is one awesome build! I just purchased a few plastic disk type sleds to see if I can make into a Cap shield. Keep up the good work!