I would use round head bolts upsidedown to attach to frame. Nuts and washers on the inside of the locked box.. Harder to steal the box that way...
Love your lever lock verses latch using pad lock...
What are you charging for one of these now haha...
I would want mine to mount on top of the trailer tongue. Doesn't bother me if it extends out on both sides at all... Also doesn't have to be wedge shaped...
Mine needs to fit two each Trojan 12VDC T-1275s
There may be a patent using the Lowes sink strainers... been done before...

Nice job...
Roy Ken