I finally found some canvas strap that was the right width, I just needed to dye it to the correct colour. I couldn't find the metal loops with the correct dimensions so I sourced some D Rings that were very close and re-shaped them using two pairs of pliers. For the metal studs I used a couple of galvanised nails, pushed them through the strap, cut off the excess shaft, dropped on a small washer and mushroomed the nail (like a rivet) over the washer using a hammer.
I made two of these straps and have sewn them to my harness, I'm very pleased with the results.
Are there any other members going down this route for attaching the JP?