I have a stainless steel diesel tank and need to fit a hatch or lid to better access to the inside of the tank. The hatch will be placed on top of the tank. I'm thinking about using a normal plastic inspection port, but don't know how it can be affected by the diesel. Any advice would be appreciate.

I would go with the plastic inspection port. Easy to fit and I doubt diesel will affect it much.

If after five years it has gone brittle or something, just replace it.

Do you really need to fit an inspection port?? That tank looks really comfortable the way it is.