So just an afterthought on the purchase of the "light engines" from Cobkits.
I had not settled on a way to mount these heat sinks and LEDs until recently a month after making my purchase I settled on the rapid LED canopy substrate.
If you do plan on using the rapid LED substrate, I would encourage you to also purchase your heat sinks for Rapid LED as you would be more guaranteed an exact fit into the rails.
Given what the rapid LED staff of straight offers for the do it yourself LED grow light Community I am sure companies like cobkits will soon offer 140mm pin fin heat sink options.
With this rail system being so new, compatible heatsink size is a serious consideration especially for people who are going into this rail system for the ability to put Quantum boards and cob LEDs on an easy to manage frame.