I built mine too. I have a 16' Corn Pro flatbed. The stake pockets are arranged so that there are pairs close together in spots. It was obviously meant for the side sections to meet at these spots.
I used some lumber I had on hand. The stakes are mostly oak, and will hold substantial loads, like logs. I put a 2x2 along the bottom. This acts as a stop to prevent the stake from dropping too far into the pocket.
I then spaced 2x4's and/or 2x6's every so often up the stakes until I got the height I wanted. On my trailer, there are three rails with about 4" between each rail. This gives sides about 28 to 30" tall. (I don't want to go measure it in the shed. )
The longest, and heaviest, section extends over the fenders. It's about 6' long. The others are all about 3-4' long. None are very light, but one old guy (me) can lift each one alone.
I wanted to use proper stake-side hardware, but I couldn't find any. Instead I connected the sections with hinges. I bought galvanized (or maybe cadmium plated) gate hinges and ground the swage off one end of the hinge pin. I replaced the hinge pin with a bolt. Each bolt had a hole drilled through it at the end where I inserted a spring hitch pin to prevent the bolt from rattling out of the hinge. The problem with this arrangement is that in order to remove a section without starting at one end, you have to slide each section sideways so that the hinge pin holes are out of alignment. Otherwise, the hinges will catch on each other and prevent lifting the section.
Two things I recommend if you make your own. First, label each section so that you will put it back where it came from. They will be easier to install if you have labels.
Second, be sure not to fit the stakes too tightly to the stake holes. I didn't want mine to rattle, so I made them tight on purpose. I found out later that they were very difficult to remove, and the hinge overlap was also a problem. Some work with a belt sander took care of that, but it would have been better to do it right the first time.
Good luck.
Loren