I stumbled upon this website for replacement RV skylights.
I figured this would be helpful locating the right replacement for a DIY project.
https://www.eztopsworldwide.com/
Remember do not use silicone, this product works great with fiberglass roofs;
https://www.geocelusa.com/product/pro-flex-rv-flexible-sealant/
Another alternative or use in conjunction with the Proflex;
https://www.eternabond.com/