There are several quality plastic battery boxes on the market.
Century Plastics builds a "no frills" box in several sizes to hold two or four GC2 batteries. The lids would need to be 'modified' to add the vent attachment. That's just drilling a hole in the top and sealing the flanged hose adapter to the lid.
Here's a link to the pdf for their boxes:
http://www.centuryplastics.ca/images...ment_Boxes.pdf
NOCO also builds some extremely good boxes, they do tend to be larger, a bit bulkier and they don't seal as well as the Century Plastics boxes. I have a dual NOCO box sitting in my pole barn because it's just too difficult to get the batteries into and out of the box when in the front compartment of my fifth wheel.