I did this on my previous Rockwood trailer. I used a HF lockable box. It was around $100 and sat perfectly on the frame. I cut two 4x6s that sat in-between the battery rails and supported the box level with the frame. Then I used 6" bolts and bolted through the battery rails, the blocks, and attached the nuts inside the battery box. It was a tight fit between opening the lid, the propane tanks, and the front of the trailer. In fact, I had to remove the propane tank cover just to get the key into the lock. Picture attached. It all worked well but the hardest part was routing the thick wires into the box. And of course, it wouldn't stop a determined thief but would deter the other 95%. I may do the same to my new 262RB, but for now I just remove the battery when I'm done camping.