If you are referring to the lids on the two lockers behind the helm seats, a flat sheet of starboard mat not work well . Unlike the aft locker lids which are flat, the forward lids are molded and in order to sit flush with the hinge area, have to be thinner (but with a lip) than starboard would have to be.
Although the old lids may be shot, if you have them, they can be patched up to be used to make molds. If you are handy and adventuresome, you could make molds and then make lids. It is a lot of work and will cost several hundred in supplies but will be less expensive than buying custom fabricated from a shop.
You may also be able to get a better price from a fiberglass fabricator if you have the old lids that they can use to make molds