Not sure what your boat looks like in that area, but I used to have a Nitro 185 Sport. I filled in that area with PT plywood covered with outdoor carpet from Home Depot. The color wasn't a perfect match, but I wasn't going to a beauty pageant! Mine was friction fit and sat on the fiberglass lip on 3 sides. It had no problem supporting my fat arse!
Thanks Bob. Same situation. Years ago, I think there was a manufacturer insert for that. Should have bought it when I saw it. Looks like I'm going to construct something as well.