That's the wonderful thing about trailers! It's easy to buy the type that suits you the best. Trucks don't offer that much flexibility. Trailers don't need oil changes or much upkeep. I must admit that I really like what I currently have, but I wish it was 5' X 10'.
On my old tilt bed trailer, the floor has a thin sheet of 4' x 8' treated plywood on top of the trailer planks. The wood sides are rough lumber I got from my dad for free. He had someone come in with a portable sawmill years ago, & he still has a number of planks stored in the barn.
My wooden box is held into my trailer with two bolts. Rear ends have one bolt attaching them to the frame of the trailer, & that holds everything in place. The front board & rear board just lift out. I use the rear board to drive stuff like the ATV in the pic. The board just tilts the bed down so I can drive up, and the trailer flops back down as I get the front. I have no need for a huge gate hanging off the end.
The sides are adequate to hold a yard of gravel (over a ton), which is more than that trailer should probably hold.
I prefer the wood sides. I buy 10-12 foot lumber & remove the front/rear boards & let the lumber extend out in both directions. It's easy to strap things down in a utility trailer like this.
More width would be better for my Kawasaki Mule. More length would be better for my old compact tractor (last pic) with a tiller or mower deck on the 3-point hitch. Bolens tractor has a diesel Mitsubishi engine!
If I wanted a lightweight box trailer, my local Powersports dealer carries the Floe CargoMax trailer (clip attached). A 5' x 8' is a little over $2,000. They offer larger sizes, too. It's a neat little trailer, too.