:it fought the spray adhesive, just not happy with it so I pulled it off

Sounds like you tried using only a few large pieces of the foam . . . .

As this stuff shapes so easily, just use a whole bunch of pieces that you spray glue to each other as well as the deck, one by one marching around the opening. Each pc only needs to be 2" wide or so by about 3" approx. that mitre around the opening and over hang just a tiny bit.
You end up with a whole bunch of little steps that are now are easy to shape and trim around the opening - as you have glued them virtually flush to it. Certainly no fighting necessary. It's best to use 1" thick foam as the sanded result will give about 3/4" clear or more, which I need for some rands I have.
Not necessary (as foam is easily removed), but you can mask the foam (only) with tape or plastic wrap or whatever process you like. Then lay up however you want - I use enough to wrap under about 1", so pcs are about 3"-4" wide by about 6" long with minimum layers of 6 lyr (probably end up with 8) of 6 oz.

The under wrap usually hangs free quite a bit on layup, so I either use lots of 9-12" long pces of duct tape over the newly laid coaming and under the deck to hold the hangers up to the underdeck or layup one side at a time with the yak on its side.
I usually lay up the 6, let setup, trim a bit and maybe reshape slightly with a heavy rasp, and then lay on a few more (maybe CF) to make a better finish - but I'm a hack - most would do it all at once.

As impact weight is often put on the coaming, it's an idea to add fillets of some nature between the top of the deck and the upstand exterior so that coaming punching is minimized.