Yes I have found that as well and that is what I want. Just not sure about cutting the trailer to make it work. What I would like is that same size of box minus the fill and vent but to be able to just mount it over the one that is there now.
I also realize about the vent needing air but only when filling and using the pump. Most times there is no water in there and would be nice to keep crap from getting into it.
Going to check some RV stores when I get a chance and see what I can find.
Would like someting like this except deep enough to go over what I have now and just screw it to the trailer.
JR Products 22132-A Polar White Large 30/50 Amp Key Lock Electric Cable Hatch: Amazon.ca: Automotive