I finally finished the trailer up. I grabbed a fresh pallet from the lumberyard down the street and cut some nice rails for the rear gate. I marked off the corners for the keyhole slots for the rails to set into, and used a drill and jigsaw to cut them out. Now the trailer has 4 removable sides and works like a charm.
Weight is a bit of an issue- I think the thing probably weighs about 30 lbs. now, but since it’s made to haul stuff I shouldn’t be too worried about a little weight. I am planning to do salvage missions for paving bricks and wood to burn in the fire-pit, so speed isn’t a huge concern. The thing seems heavy duty enough to carry a lot of weight, so I should probably test out the capacity, and see how she rolls!