i would take the size of the hole add 30mm then make it into a square it is better to take gelcoat of but it must just be rough so the mat can bond to it properly. (glass fiber on a smooth surface will delaminate off over time)
I would use a flapper disk on a baby grinder 80 grit
With foaming it is important to do it at one go as foam forms a skin, with the size hole you have dont worry to much as excess foam will come out the hole so just mask it off with tape so does not stick around hole the foam that comes out once it sets you can just cut off.
if you do bit by bit you will leave gaps and water will find its way in. As foam come out the inspection hatch hole you want to block it for a few seconds to force the foam into every nook and cranny in the compartment
