I am pretty sure I saw just the thing you are looking for at a place called Metro Trailer on Nairn Avenue here in Winnipeg. I only took note of it because the belly band on our surfside
is in terrible shape, all painted over and missing in spots. We aren't planning exterior until later but I have been scoping out places to buy parts, fiberglass mat, resin and gel coat. Been fairly lucky. Will be getting hinges, brake parts, lights
etc. from them as they seem to have a little of everything that I never see at the RV places. The insert they had was black and looked to be the right size. Maybe they'd ship to you.