You may have to throw that perfectly good one out and buy two more that matches haha...
I got two batteries here in my back room sitting inside two of those plastic PORTABLE FILE BOX cases by STERILITE (WALMART)

You could probably contact the dealer or factory that made the trailer and find out who they got their battery boxes from (Probably NOCO)... I bet they get orders for multiple batteries in their sales...
The two big companies I can remember is NOCO and MTS
Roy Ken