Starboard (and many other plastic panels like Marine Board (which is basically the same thing, with a different name- it's all UHPE) can be formed using heat. If you call a supplier or the manufacturer, search online or watch videos on YouTube, you'll find the info you need WRT temperature used for forming shapes. IIRC, less than 300°F and time to conform is about right.
You could make them from Fiberglass, too.
www.boatworkstoday.com has at least one video showing him making a hatch cover and it would be done in a similar way, using thin bending plywood and wooden pieces that have been cut to match the curve, glued to the backside of the form. He used particle board with a Melamine surface and it's available in 1/4" thickness, but I think I would go to a big box store and buy a small sheet of laminate, use contact cement to attach it to the curved panel (to prevent the resin from sticking) and by making it longer than you need, you can cut it to fit.