I don't think its solid foam all the way to the keel. Prob the foam with a stiff wire. Maybe all you need to do is cut out more foam to access the interior.
Just chisel out that area of foam towards the area you want to access. I did this on a Sunfish to get to bridle eyestraps. You will loose minmal structural support.
This won't help now but for future reference it is not solid foam all the way across the deck behind the 'splashguard'; you happened to hit one of the two longitudinal foam blocks. At this point I would carve outa foam cylinder under hole. You will likely be close to one edge of the block. I would then cut through to the side so you can use the port as intended. Late fyi, if you tap the deck you may be able to tell where the blocks aren't.
If the hull is left outside I have had condensation from dew that shows exactly where the foam blocks are located. A digital laser thermometer can be used to check hull temperature differences and find the blocks.
I would put a port in as it can be used in the future and you can decide later about the foam remove. I use rivets on ports.
I have always preferred the cockpit ports in regular Sunfish but Mini Fish I am not familiar with the cockpit construction.
A picture of the whole deck would be helpful for future reference on the forum where not to put a port on a Mini Fish.
I have several pictures of the Minifish - I can upload them all at some point. I'll try to dig out a little bit of foam and go ahead an install the inspection port. Thanks for all your ideas.
Tonight I probed the foam with a wire, and then carved out the expanding foam and sure enough it was maybe 1 1/2 to 2 inches thick. There is a main flotation foam block running along the left side of the port but I should have enough room to reach inside and install the backing hardware to install a ratchet block and cleat.
I'll take some pictures tomorrow when I work on installing everything.
Ok, so carving out the bedding foam was a success.
I then was able to reach into the inside of the boat, and install the backing hardware for a mainsheet ratchet block (I used the Holt Nautos 57mm one from Intensity Sails). I plan to install a H150 cam cleat on the cockpit wall, but didn't have the right screws available. I did install the inspection port, too.
Other upgrades: I installed a mast cleat to tie off the halyard.
And then took off the old wooden tiller extension, and replaced it with a Ronstan Battlestick with a rubber universal joint.
And a quick-release for the gooseneck, but I forgot to take a picture of that.
I did get around to installing a Harken H150 cam cleat on the wall of the Minifish cockpit. I re-used one of the holes from the old hook, so only had to drill 1 new hole.