Plastic tanks have their advantages, and their disadvantages.
Cecil, you could get 2 pieces of thick plexiglass and bolt them to the bottom of the tank (one outside and one inside) with gaskets between them to seal the bottom of the tank. Then drill a smaller hole thru them for the smaller fitting.
I do not think I mentioned Plastic Tanks, and Plexiglas would be a disastrous solution for this application
