I have purchased LED's for a variety of applications over the last few years. My two go-to sources are LED wholesalers and superbright LED's, both have extensive websites. I will also admit to falling victim to a seller on eBay because of price, his product was a piece of Chinese garbage. There are many types of LED's intended for marine applications. Underwater lights, bait well lights, deck lights and so forth. I think it's going to be way easier to find a light then to manufacture a mount though. You could certainly fabricate a simple "S" bracket out of aluminum flat stock, mounting it to your mast is up to you. If it's below the boom you could use large SS hose clamps from the box store, above the boom you could drill and tap screw holes as long as you used something like tefgel when putting the SS screws in. Wiring and switching is another issue all together.