One note is to do a bit of research on what type of sealant to use on the deck plate. There are two possible concerns:
1) Adhesion to the water tank's particular plastic make-up
2) Affect on potability of water
(Of course neither of these were specific concerns of the original poster.)
I can't recall right now how these are typically installed on plastic water tanks on boats (in fact, I'm thinking that this type of tank doesn't usually get a clean-out port; maybe that's why - sorry, I can't remember).
A rubber gasket may be a possibility.
It's a slick mod for how the original poster is using it