The original seats would be best as I could make a master pattern to manufacture later from. I'd make in two parts, the base and back, have a zip up the back of the seat back, a flap at the hinge (either velcro or another zip) and another velcro/zip system underneath to keep the base on.
But if the genuine covers are a good enough fit I could work from those but the end result would be a fit as per theirs. So any bagging/tension would be replicated.
I'm in Seven Hills, and available all day during the week. I can organise some time with the machinists to inspect & measure in advance and meet there. He's been making my wetsuits for over ten years and apart from working suits (I'm a commercial diver) which can be trashed in a day or two I still use the first ever suit he made me (apart from being a bit snug these days). Choice of colors, red, orange, blues, silver, white, black, pink, purple, greens and black, matching or contrasting threads.
