Don't look for those overpriced boxes, just BUILD what you need like I did. Both sides of mine are done now. The Passenger side has my lithium battery and storage space. I did that like a full year ago. The left side now contains pretty much my entire water system including the hot water heater. This keeps everything but two lines OUTSIDE the trailer to avoid any water damage from surprise leaks.
Simply buy the matching tandem style fender for the top, round back or angled flat to match what you have for fenders now, cutting one end off to get the length you need. Simply buy and bend to shape the bottom and door from 1/8" aluminum sheet. You don't even necessarily have to be able to bend it, you can just cut it and use lightweight aluminum angle and pop rivet it together.
I only bought one fender, and got enough to do both sides. I originally had lead acid batteries and put supports underneath that side, but the lithium probably didn't need it. I can now stand on either side when I need to because of all the support the box shape provides.
If you are a registered member, you can see the images of mine in my gallery:
memberlist.php?mode=viewprofile&u=1304Or Dig back thru the posts I put in place as a build history here:
viewtopic.php?t=70777
Posted: Tue Oct 25, 2016 8:19 pm