Newbee to the forum and have learned lots of great info from here, so I thought I would share my solution for a secure battery box for my Rpod.
Like some of the other Rpod posters on the forum, I decided to go with the HAUL-MASTER
2.31 Cu. Ft. Steel Trailer Tongue Box (https://www.harborfreight.com/231-cu-ft-steel-trailer-tongue-box-61602.html)
I had to make some minor modifications to make it fit by turning the propane tank around and making a small cut at the bottom of the propane cover. I have to remove the propane cover in order to access the lock on front of the box, but it fits snug between the propane tank and front of the trailer.
The construction of the box is a lightweight steel and the lock is a cheap cam lock mechanism. I would not trust the security of this box by putting expensive equipment in it such as lithium ion batteries. However, I am using less expensive deep cycle lead batteries. I estimate I could get 3 batteries into the box, although I currently just have one. I also added a bettery cut off switch to the box itself.
Overall, an affordable option that does the job at a fraction of the cost of other secure boxes I looked at ($100 vs $300 plus). I give it 2 out of 5 stars on durability, but it does fit nicely on the Rpod trailer tongue.