Don't over think this... I've seen these built from 2X4's, and PVC pipe. Mine is build from 2x4 and 4x4's, and I bought 4 casters from Lowes. Measurements, really are pretty simple. Look at your trailer. The weight of your ski is at the engine, and a bit forward for fuel. Mine measures about 6 feet in length, and 3 or 4 feet wide. I put down one centerboard, with carpet. I blocked the back casters so they would not move, then I put my trailer in front, and started releasing the cable and pushing the ski off to the wooden flat wood frame (you need two people). Once the center of the ski was on the center board, I started pulling the trailer out, allowing the ski to sit on that center board. Once it was starting to transfer the weight, I had my buddy take a couple 4/4's and use them as wedges. Once the ski was on the center board, I got a measurement from the wedges I placed, cut some 2x4's to run the length, carpeted them, then put bracing down on my wooden frame, to stabilize it. I can roll it around my shop, roll it out side to wash it, and work on it. The ski is the GTX, 12 feet long. Don't over think this. You don't need a bunch of metal, welding machines, and winchs... Pull them up on google, look at images, ... it's a really easy to do it your self project.
