I'm liking the bed liner stuff.

Boxes are subject to a lot of vibration, so the coating needs to be flexible and tough. The road is a very harsh environment, especially in the rain, when all the gook on the road ends up on the sailbox.

I'd be concerned that the bond with the wood might fail - you'd need to stay on top of repairing any penetrating dings in the coating.

It just doesn't win in the looks department. Gotta look good to go fast, even with the sailbox.