I have several "inspection ports" on my two 220 litre aluminium tanks.
They are just aluminium plate bolted to the top using stainless steel
machine screws taped into the aluminium of the tank.
I had a fuel filler cap welded to a plate on each of my tanks so I can check the fuel levels.
I had a look inside the tanks when I took the plates off. All I could see were aluminium shavings from where the tank had been made.
If your tanks are made of thick aluminium then it would be very difficult to cut in place. I ended up using a hole saw and a drill press. The big holes need a very low speed.
If you have a look at my blog from a few months ago you will get some idea of what I have.