I cut this message out of another thread if you want more detail, it's located here:
Here are some photos of me cutting the top out of a keg to try this idea for better access to the inside of a fermenter.
Basically I cut a small hole in the fermentor and used the "cutout" from a converted keggle as a lid.
There have been threads re the simplest keg cutting jig... mine is more complicated. It's adjustable horizontally and vertically. It rides on the rim of the keg, so it doesn't drop when the cut is complete.
Had to make some adjustments (2x4 clamped behind the angle grinder) because I wanted to make this hole pretty small (about 9" in diameter). The cuts shown are before any cleanup took place.
The pressure cooker "seal" I found didn't work so great so I bought a very large silicone o-ring from McMaster. I cut some plywood rings to get even pressure all the way around the "lid". I tested it with c-clamps, but ended up finding some other clamp that I was able to modify to work much better.
Before filling the fermentor, I clean it and sanitize it with steam by boiling a gallon or so of water out of it on a turkey fryer.
The pressure cooker "seal" I found. Then just cut some plywood rings to get even pressure all the way around the "lid". I tested iw c-clamps, but need to find a better way to clamp the top down. Similar to GreenMonti, J-bolts may be the answer.