I know the instruction was to not suggest just buying them, but...

Just buy them. For the single axle mounting, I just got some trailer rollers, scuffed them up, and bedded them into the square areas with 5200. Then mount to the trailer the roller axles as usual. They haven't budged in years. When I'm not trailering, I made simple stands with 2X4s with brackets and mount the cradles with the same roller axle as used on the trailer. The thing with the back of the trailer that most around here do is have double rollers in the back - they make sliding the boat on and off the trailer easy. Once the boat's on the trailer, lift the back of the boat up and drop your cradle onto the double rollers - the square mount fits between the rollers holding it in place, the whole assembly pivots to match the hull. Simple, easy.