Sealant
Aurel, I removed and resealed the inspection port on my 20 gallon holding tank using 3M 100% Clear Silicone. I've had no leaks and there has been no trace of and odor comming from around the inspection port. You can find this at any home improvement store in a small tube for around 2-3 dollars. It makes a great sealant, its removable, cheap and it cleans up easily.