I had a pop up trailer that I converted to a wood deck utility trailer in the 80's using 2x12 planks with low fixed sides. it was 12ft long and the tailgate lifted off. I used that to haul a 14ft flat bottom Grumman jon boat for over 10 years. I had bought a boat trailer to replace it but found it easier to use the flat trailer. I made up full length wood bunks with bolts that protruded from the bottom of the bunks that I'd drop into holes drilled in the deck, I'd attach them with a wing nut at each end so they didn't float away or stick to the boat.
I'd leave the tail gate at home, mount the two bunks and slide the boat off a same height cart I made for it, then once at the water I'd just back down the ramp, with the motor tilted up, and push the boat off the trailer. I had a boat winch up front to reload it when I was done. I carried a can of spray silicone to keep the bunks slippery and water proof. It was fast and easy and I didn't have to fight with a boat that didn't want to line up with the trailer, the bunks stuck out the back about 18" and when on the ramp they'd be in or just at the water's surface.
The boat was light, with a 10hp motor, one battery, and the usual gear. Maybe 600lbs tops in all.
It was light enough that I could load it without the winch if I had to.
Best of all, it was easier to tie down on that trailer, one strap and the winch left hooked to the bow and it wasn't moving. There was no way for it to slip or bounce sideways and no worry about it not sitting straight on the trailer. The bunks were spaced to fit just outside of the lower strakes on the hull.
I even used that set up to pickup a few other boats I had over the years. The fact that it was the same level as the roller stand I made for the boat on wheels meant I could wheel the boat into the shed in the off season out of the weather and there was no trailer tongue in the way in the garage.
