I have played with the idea of building a general purpose cat trailer based on a flat bed. Start with a welded aluminum utility trailer
(make the top of the box even with the fenders)

Add mast supports and deck the top with wood, to carry the boat. The "box" is now a big cat box. I determined that a 8x10 ft decked area could carry any boat I own.

The exact configuration would depend on who was building it. It is a lot cheaper to modify an existing model than have a custom trailer built.