Another option in case none of the Torklift models work out is to have a custom box built by a metal shop. That is the route I went recently when upgrading from two batteries to four T-105 golf cart batteries. I have several friends that lost their batteries to theft by the meth heads looking for a quick buck. They also had to deal with the damage to the trailer.
The shop that built the box for me is KSH Marine in Lake Havasu City, AZ. They did a great job at a fair price. You should be able to find something similar in your area but I do believe they ship in case you want to go that route.
KSH Marine Web SiteI believe this is what the front end of your new trailer looks like .....
Trailer Front End PhotoOne challenge your trailer presents is the sloping front end. That could cut into the upper area of the box. Some things to keep in mind are having enough room to get the lid open and to be able to mount and operate the spring bars for the hitch. I ended up resting the box on a couple of 2 in X 2 in sq tubing rails to clear the spring bar clamps and to support the weight of the box and batteries. Upward clearance was not an issue since my trailer has a vertical front end. You may need a sliding lid or some other creative approach.
One other thing to keep in mind is that if the box is metal there must be a enough free space so the positive posts or uninsulated wires are not at risk of contacting any metal of the box. I used a wood framework between the batteries and the sides of the box to keep the batteries from shifting.
As far as venting, I went with a series of 3 in round louvered screened vent covers.
Vent Covers from Vent My HouseBe sure an come back with pictures of what you ended up doing.
Nash 24A Travel Trailer by Northwood Manufacturing (6,800 LB)
2,000 Watt Prosine Inverter, 540 Watts Solar, 3024i Controller & IPN
Chevy Duramax (LLY) Extended Cab Pickup