I got a new wood wagon last week and it needed a side kit to safely do what I wanted it to do. It had a 1/4" lip that comes about 3" off the floor so I used some washers to add the necessary spacing between the stakes and 2×12 sides. All 4 pieces come off separately and fit well. Only thing left is to brace the front bulkhead to protect the winch in case I really have to lay on the brakes. I make plans as I go so it took a while and has some flaws that I fixed on the go..lol.. I could build a better one in half the time now.
I take it you mount a battery where the winch connections are? I bent up a piece of 1/8" and hung a battery on the other side of the jack like where my spare tire is. Wired it up so it charges off the truck. Couple other idears. The winch on the trailer is inside the box. I just stack wood over the top of it. So far has held up fine, even when loading 20ft logs on it.
I haven't used the winch yet but my dad had a portable power pack that he used. I want to run it off the truck though. Great idea.
Too bad you don't have a router...could have avoided the need for those washers. Looks good though. I don't think you could hit the brakes hard enough to worry about the winch if you sink a few lags in the ends of those 2x12 instead of those screws.