I have a plastic Mk2. I managed to squeeze into the gap to put nuts on the back of bolts through the stemhead fitting. In the process, I beefed up the area with epoxy using a plasticine and cardboard former.
I also epoxied several places where impact on the outside had caused damage on the inside to the join where the top surface of the front bouyancy tank meets the hull. From the outside the damage could be seen as small star crazes, while from the inside, even with a bright light, it was only observable with a bicycle pump and washing up liquid. This very slight damage was enough to fail the pressure test.
It was a rather tight squeeze and I could only gain access by removing the mast tabernacle moulding.
I am not particularly thin although I did have to breathe in to get past the raised lip of the front tank and it wasn’t something I would want to do every day.
I found it to be slightly easier when the boat was upside down on trestles a couple of feet off the ground.