I cut open my Moeller
Marine polymer tank to install an
inspection hatch from Tek
Tanks UK this last
winter.. The bolts need to be inserted from the inside of the tank and the nuts are screwed on from the outside. I modified the bolts so that it is possible to tighten the nuts without turning the bolts.
Hatch is leak-free so far.
Quite scary to cut a big hole in your tank without being sure it won’t leak.
PM me if you need more details.