I must put my $.02 in here.

First the idea is novel, as nobody has pointed out so far, completely unnecessary, unless you go with 1000 (3w) diodes but very novel.
Second, epoxy is decent as long as you scrape both surfaces before use. But I swear, I don't wanna hear you using epoxy anywhere near this heat sink.
Third, use the right parts for the job. If your using copper tubing, use copper fittings, there are TONS of online stores that are super cheap on parts.
Fourth, Use solder on your pipes, don't use the stupid caulk, high pressure or low pressure, use the soder, after each piece is made, submerge it in water and turn an air pump on it, pressurize it, and make sure there are no leaks.
Fifth, use a pipe bender to make your coils, coil them around a 2" pipe - If you feel a pipe bender isn't needed and you choose to do it by hand, make sure the coils are super tight, as close to air tight as you can get.

Sixth, and this most important, not only for your personal safety but for your freedom, DO NOT 'half ass' anything on this project. Water and electricity is one area of DIY that NOBODY half asses for very important reasons - ADDED to this reason, your using metal tubing, which means your creating one large electrical conductor, the fact that you haven't taken condensation into consideration is a HUGE red flag.

Seventh (yes, there is more) - what kind of boards are you working on for your LED's? Which transformers and lights are you using?

Finally - I've been working on a design for an LED light panel for a CO2 grow that should give enough penetration to any grow no matter what strain your dealing with. As soon as I'm done with the design it's off to get the patent. If you can create something that would be of use for me for cooling, that would be great. I'm planning on pushing this idea to large companies coming into the commercial end of things, the heat-sink could actually serve two purposes in my application, however would need a slight modification for the type of grows I'm considering... At any rate, don't half ass it, there could be possibilities.