Can't seem to find the discussion on this. After I cleaned the sludge out of the intake side screen on the water pump, I thought I better get inside my tank and clean it out. My tank is plastic. I have a 4" hole saw. Can I just fasten a round 4" access panel to the top of the tank? What should I use to seal around the fasteners? I think I might use machine screws with oversize washers.