You are off to a good start! If you have a good tight seal on the wood to the frame you will probably be OK, but you could run a bead of silicone along the joint to help keep any water from getting in too.
Regarding HF trailers, For my second "Teardrop", I used the super small 3x4 Harbor freight trailer ($140). I wanted the wheels further back so it worked out great to weld in some angle iron to the front to make the trailer 3x7. Since it was only three foot wide, I got some spacers and raised the frame off the axle 2" and lowered the fenders and viola' I was able to put a 4x7 sheet of plywood right on top of it, and build over the wheels without them getting in the way. The same concept would work to build a five wide on a 4x8 trailer too.
It ain't sexy, just an air conditioned and heated hard sided tent!
Good luck on your project and be careful with that paint!
