Modeling Your Build
A suggestion: A Hack that works...
You strike me as a person not in a particular hurry. Order up the most appropriate and best priced cheapo computer/fan heatsink you can find on ebay. Look for one you figure has enough surface area for the COB, and that the cob holder screw holes will thread into the space between the fins, you can bend a few fins to to make sure the screws grip tight, use longer 3mm screws (hardware store) and even cheap epoxy to make sure the screws are secure. You might be able to put your COB holder on this way and it would greatly expand your options and, lower your costs (I've done it myself) and allow your COBs to safely run at 50 watts. Go on ebay and blow 20 or 30 bucks, buy a cheap say, 20-30 or even 50 watt enclosed driver (see the link earlier in the thread) and one heatsink/fan I assume you'll order your COBs of choice, driver, holders etc (after finding and confirming your requirements). Round style heatsinks with radial fins work best for this purpose. Yer gonna build this fucker anyway, ya just haven't quite figured out how exactly. When you build you can use the leftover driver for a for cheap ebay 5000K COB veg lamp later, so nothing is wasted, unless you screw up on the heatsink. A cheap COB will do and often they sell them with the driver, tip use 2 chepos driven in parallel for extra coverage in your future veg lamp.
What were gonna do is model yer perspective grow light with 1 cob, holder and 1 ebay heatsink/fan, build small for practice, before committing and see if the idea will work for low cost. See if you can put yer cob on the heatsink like described above, an ebay 12 volt power supply module is cheap and since yer gonna need fans anyway get one big enough for the full lamp (add up the .25 amp (25ma) or .3 amp (30ma) or whatever) of the fans and buy a supply with a bigger number, scroll down and look in the specs. Most of these supplies are adjustable to a limited degree and will probably allow the fans to run at 10 volts very quietly. If you get this setup to work and if everything looks ok, order up the rest of the heatsinks and wait for the slow boat from China. Dollar store light bulbs can get ya going and your single COB lamp will be very useful until the rest of your heat sinks arrive. You'll be ready when your parts arrive for final assembly and it'll give you a chance to find the other things you'll need. Avoid Home Depot or hardware stores for angle, unless ya wanna be butt hurt, look for a local metals supplier that sells extrusions for your 1 x 1 aluminum angle, ask about heatsink extrusions for me while yer there.
You can dip yer toe in and try the waters, also using these heatsinks would allow you to run yer COBs at 50 watts, if ya wanted. The COB would be about 35 efficient at that current and the back of the cob should emit an estimated 35 watts of heat ( you know how to calculate this if you wish) into a 80 watt heatsink working at 60 watts capacity because the fans run slower than originally specified, for cooler COBs up the fan speed. Play around, learn a bit, have some fun and do some hands on.
Hope this helps, cause I'm try'n to lead ya down the path of perdition.
