I am in the middle of doing the same thing. Did you install the plastic box from the inside or outside? in other words is the shell of the trailer between the cast door frame and the box or does the lip of the box sit on the outside of the trailer skin? Did you use the rivets supplied with the kit? My other box is attached with screws.
I installed the box from the outside; those batteries weigh a lot and it's more secure that way. I just picked the door and frame up used off eBay, so I didn't get rivets - I just used stainless steel sheet metal screws.
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