Jerry,

I am not fortunate to have a trim router but I have a regular router. Any reason not to use that and a 1/4" bit to cut the hole? I don't think the top skin is thicker that 3/8's and probably closer to 1/4." I have talked to my friend regarding borrowing his grinder, and he has some 4" blades for it. I think I am in good shape tool wise.

I don't think I understand what you are suggesting to do in the void area. My plan was to fill the entire area with filled epoxy and keep stuffing until I had the void filled and then build the deck area up to match the deck using glass mat. I I have a couple of choices for filler in the void including glass fiber, micre balloons (I don't think it's a good choice) or one of the other solid fillers.