I converted my 9x5 utility trailer. I wanted to keep weight to a minimum and I needed to make it removable so I could use the trailer to haul other stuff.
I filled in the sides with sheets of roll flashing, pop riveted to the steel angle trailer frame. They don't need to be super strong, just fill the holes so suff doesn't fall out.
I added a bolt-on top made of two panels hinged along the center line of the trailer. The top is made of 1x6 fence rails open spaced at about 12" o.c., these are bolted to a frame made of steel U channel. The U channel is typically used to mount electrical junction boxes. It comes with all kinds of connection bolts and mounting hardware that can be adapted to the trailer. It is found near the conduit in the electrical aisle of most big box hardware stores. The U channel frame is bolted to the trailer and easily removed. The fence rails are carefully placed to align with my rubber and provide walkways. I cut the weight almost in half by using rails over solid surface.
Some things I learned. All nuts and bolts need to be made vibration proof. At least use Nylock locking nuts or stuff will just shake loose. Screws or lags do not work, only through bolts. I used a lot of low profile T nuts for most of the connections.
I achieve my water proofing by placing a tarp over the load before closing the lid. The tarp is big enough to roll down the sides and tuck under a bit.
Most of the time I want my trailer to be more drip through than weather tight. If I load wet stuff and keep the top covered it has a chance to dry out on the drive home. The tarp is a cheap easily replaceable solution.
My top panels overhang the trailer by about 8" front and back, about 6" at the sides. I took section of PVC pipe, notched them to fit over the fence board and attached them front and back for a smooth edge to drag the boat over. I added a bunch of eye bolts to the perimeter of he trailer so I would have a lot of tie down points.
One challenge with the utility trailer is that the platform is so high that is is difficult to get the boat onto it. I don't often try to do it from the water. I find it easier to drag he boat to dry land, park the trailer along side and lift the boat on from the side. If I roll the boat onto its side I can rest one edge on the trailer and lift the opposite side to slide it on.