Epoxy? It's like any other heatsinking problem: keep the gap thin, and fill it with something that's reasonably thermally conductive.
One of my former employees works for a big lighting fixtures company, and dropped by for a beer on Monday night. He gave me a couple of top-secret white LED assemblies, each about a square inch of aluminum with a blue led array and fluorescent phosphor layer. At, say, 15 volts and 1 amp, they look like welding arcs... you see spots for minutes after. He claims they're twice as efficient as CFs.
The fixture design problem is to get the heat out, cheaply.
Well I had 9. I used a copper plate to go to the LED's. I had to insulate them so I spaced with diamond dust and used slow hardning ceramic thermal epoxy. The plate goes to a peltier and then to a heatsink and fan. I used a computer sink and fan.