A few updates:
1. As some who is new to the world of trailers, I've learned a lot. First, and perhaps most importantly, the trailer sitting in this position (see above) as it's natural resting state is not good. I added a storage rack in front of the box to shift some weight onto the tongue, and had the tongue extended a bit. A big improvement in balance and towing. It used to get squirrely above 80 km/h, now it seems smooth up to 100.
2. I don't know how to use a measuring tape. When I bought it, one of the selling features was that the axle had the right (5 x 4.5") bolt circle for matching Explorer rims and tires. It turns out it is actually 5 x 4.75. Oops. In my defense, I knew that the tweaking the tape measure to go around the hub would change the reading, I just misjudged how much. Apparently my 'eyecrometer' is only accurate to .250 of an inch. (I also learned that they make a tool to measure the bolt circle of a trailer. So I'm not the first to make this mistake.) On top of that, it also turns out that this is some kind of weird home made axle with 1950s Chevy front hubs as axle hubs. I tried to service one and found parts hard to find and very expensive. A new axle is set to be here tomorrow. Hopefully I measured the spring mounting pads properly!
3. I finally got to work on the lid. I'm replacing the lip you see on the picture above with one that is beefier, and then adding a hinged top to it. In a way, I suppose I'm trying to emulate the look of an M416.
I'm taking it on a 600km one night camping trip this coming weekend. It won't be completely ready, but it will be a good opportunity to see how things work as designed and an opportunity to work out some kinks before the whole family uses it for a week at a time.