The box doesn't have a bottom so I added L-shaped aluminum to the bottom edge. This will provide something to attach the box to and help protect against water damage.
I used big washers to keep the weight of the box from pulling through the heads of the screws.
I waterproofed the joints with metal colored silicone caulk.
Jared took these pictures of me attaching the box to the frame. I used #10 hex-head screws spaced every 2 inches.
We made sure to give the metal manufacturing place the old side from the trailer so that they could cut the opening for the water heater to the exact right size. It was really easy to just slide the water heater back in and attach it using the old screws. I used heat resistant metal caulk (rated to 400 degrees).
I wasn't able to figure a way to make a new door for the water heater so I just used the old one. It attaches really easy and has the right venting. It doesn't match the box, but it does match the rest of the trailer and adds continuity.