Update for anyone interested! The trailer tongue box I bought fits a standard TrailerAid like a glove:
… so that takes care of all the road-safety items that were looking for a more permanent home. Hopefully it mounts to the frame without issue in the spring.
And as for the gate at the rear, Horsemen’s Pride does a lightweight plastic gate that I’m ordering-in and will mount to the frame with a bracket and piece of lumber which will also help to reduce the gap as this gate is 6" too short:
A friend of DH’s does custom welding on the side, so I’ll have him figure out the lower chest bar mount. And then the rest is all going to be maintenance-related as the trailer is probably over-due for servicing at this point.
All the modifications thus far look like they’ll run about $1000 total (how did everything add up so quickly?!), but in today’s market, I like to think I’d easily get that back (and then some) on top of the original purchase price if I were to sell it. At least that’s what I’m telling myself as I hand-over my Visa card