Well guys I had a chance to test the 60w multichip with the heat spreader, and it actually did help temps. A lot- full load with fan on full, before was ~67-69C, and now it's more like 46C. I used a 1/4" plate of copper (C110 alloy, pure electrical copper) and it ended up more like 6mm after machining. I suspect that you could use bar stock as it comes from somewhere like onlinemetals, maybe with a light sanding to take off any burs. I used arctic silver (silver) epoxy to glue the heatplate to the heatsink and then arctic silver alumina paste between the emitter and the copper plate. I didn't pay enough attention while gluing the spreader on, though, and it ended up off center! @%$#%(!!!
Rick, I am going to use thermal switches to protect my lights. They're like $1 each on ebay and can handle 5A. It's basically a preset thermostat, so when the temperature exceeds 70C, they switch off until it cools down 20-30C according to the listing. I plan to put one on each channel directly at the emitter. Another option may be to use 3 @ 70C and 1 @65C, so maybe a white channel pops off and stays off if it gets too hot, then the whole fixture will cool down somewhat but likely not enough for the 65C channel to kick back on. You'd notice your tank bein too blue, I think.
http://www.ebay.com/itm/160863183164...84.m1423.l2649
edit: One more thing we all need to keep in mind is some headroom - if your cooling solution at it's best can only just keep it under the limit, how much dust will it take to cook your chip?