I think your best bet would be to find a general-purpose rugged plastic box that reasonably closely fits your genny. The Contico brand at Wal-mart comes to mind.
A specialized box for your genny would probably be pretty expensive if they make one.
I made a simple wood box caulked all the joints, and have a hinged lid with rubber seals that pulls down tight which keeps the gas smell inside so I can even put it in the back of my Suburban without stinking it up with gas fumes. This box is for a Champion but would work with any genny if you adjust the dimensions.
Steve