Demeter, I use plumber's gasket material 1/8" to 3/16" and have never had a leak. It is easily replaced, there is no clean up and no chance for stray pieces falling into the tank upon removal
. It is easily inspectable since the material will begin to crack as it ages. If the inspection port is a screw in type (as in water
tanks) you could use a large rubber o-ring gasket coated with vasoline around the base of the internal threads that seats against the outer flange. These o-rings are available at any good hardware
store. Otherwise, the plumber's gasket is your best choice. Good luck and good sailing.