Here’s best one I’ve seen:
https://www.ninjette.org/forums/showthread.php?t=117957
Sell your original bodywork on Craigslist & eBay to fund your project. Or better yet, find crashed 250 without bodywork for project.
I made Cafe-racer-type seat for my VF500F 30-yrs ago. It’s very easy to make from scratch:
1. Lay out seat-base form over bike with non-corrugated cardboard, account for thickness of foam and vinyl-wrap
2. Lay down several alternating layers of fiberglass & uni-directional carbon-fibre using epoxy resin, up to 5-layers should be plenty strong. Attach mounting brackets
3. Sand rough spots out, lay down one outside layer of woven carbon-fibre and gel-coat
4. Sand and polish
5. Lay down foam of desired density. I prefer closed-cell foam
6. Take to upholstery shoppe and tell them, “here, cover this” in colour pattern of your choice.
This project is also lots easier if you’re able to cut metal with precision. Then weld or braze the pieces together