I've seen a few knock offs made and thought about making one myself. Some looked better than others, and I figured I could do ok at it being an ex boilermaker and having access to some machining gear. After loking into it more I decided to give up and bought a friction drum instead.
The big decider was visiting a specialist yacht winch rebuilder. The guy has been in the game his whole life, and his father before him. His dad owns the patent to a few styles of yacht winches. I was chasing down a good rebuilt used self tailer, 2 speed. We got to talking, the guy sure is passionate. He ended up showing me the innards of a few different makes of winches, and we spent about an hour going over it. Yacht winches for the most part aren't designed to take shock loads.
It's worth noting that a dynamic fall of a block (especially without foliage) can easily hit 5x its own weight in shock loading, meaning a half tonne piece can max out at 2.5 tonnes. More important than the weight, is the way it's applied to the winch. Failures on yacht winches in severe gybes are not uncommon. Most of the winches made are unsuitable for the task. The large expensive harken is suitable, but still not as bomb proof as the hobbs style devices. Yacht winches are also not designed for paying out large amounts of rope under load. The rope is usually dumped mid tack after the sails have emptied of wind. Small adjustments are made under load with one hand on the stack of coils, and the other on the rope. Winch over-rides are a common complaint of GRCS users. The harken winch used by Greg Goode also has the self tailing mechanism modified to accept larger rope.
If you've got need for a device like this, you are better off buying than building. It will cost you more than what you think to build it by the time you include stuff like having it professionally painted, buying heavy duty load straps, having your used winch rebuilt and tested, etc etc... and will have no resale value when you are done with it. A bought one absolves you of liability and can be sold once you are done. It will pay for itself many times over if you do have need of it, and in the big scheme of things you could probably go and do a tree job and earn enough to pay for one in the same amount of time it would take you to build one.
Shaun
