I have the battery box that is in Chuck's link. It's great, but (always a but) it doesn't allow for much clearance to lift batteries in or out of the box when it's installed in the front compartment of a fifth wheel. I found that two GP24 tall battery boxes are much easier to handle and to get in/out of the front compartment.
https://www.batteriesplus.com/produc...SABEgIia_D_BwE
What I finally did with my two GC2 batteries was mount two GP24 battery trays in the compartment, build a lid out of coroplast and vent that lid out of the compartment. Venting is probably "overkill" since the compartment bottom is open to the underside of the trailer.