I have been working on/ modifying some vests for me and my buddies lately. I have used a lot of nylon latigo, and nylon boat winch straps. The winch straps are about the same as the seat belt. I like using both. They are both super tough, and flexible. I use the latigo in the hardest hit areas, and the strap in the other areas. My suggestion, don't build your own, just cover what ya got with the seat belt or latigo.
This one has latigo on the bottom, and I added the strap material up on the sides after I took the picture. Still flexible, and didn't add a lot of weight to it.
My buddies wanted some extra protection on the bottom of this one due to some holes being poked through it. Nylon latigo bought at TSC.
Not the prettiest, but no more holes.
I just got another bulldog, so I may try and build a Latigo vest.