: Cutting that big a hole in a plastic boat without renforcement may cause
: structural problem. Rotomold plastic boat is hard to glue anything onto.
: Any opinion, guys?

One of the posters here, Ian Johnson, implies that polyurethane glues will bond to roto or poly when following the appropriate manuf. instructions(promoters etc.) If so, you could build a flange (thick or carbon fibre-therefore stiff) and glue(the poly) it from underneath. Add gasket to the cutout roto hatch and cinch down w/ your method of choice .

-mick