That's a fantastic trailer (I have the 64A). While I love my BAW trailer, here are a couple of improvements that could be made:
1- bolt-on stake pockets for sideboards to help in hauling bulky, but lighter loads like you will be hauling. Stake pockets could probably be easily made out of tubing, even PVC. Now that I think of it, it wouldn't be too hard to build a couple PVC side-boards if I needed them. If I was doing that, I'd probably start with 1" PVC couplers (2 or 3 per side), attach them, and then I'd have secure places to set in place side-boards made out of 1" PVC.
2- bolt-on pieces to go between the front of the wheels/fenders and the frame, to create a deflector if one were to ride by something too closely. Just like you see on the Surly Bill trailer.

I used corrugated plastic for a floor on mine. I found that two layers is pretty solid and fit perfectly in the space. I don't have to worry about it getting wet or messy because it's impervious and easy to clean.