I made a blog post about how to make Chip-on-board (COB) based LED fixtures (here is the link). I included materials list, step-by-step assembly details (maybe too much for most people), information about driver selection, and measurement data using PPFD.
Also I came across a couple of useful youtube videos, so the links are provided at the very end of the post. I started DIY COB LEDS with cheap eBay COBs, but the video provides the reason why these are not worth getting. Actually I think they are not worth using even if they are free.
There are newer generations of COB LEDs (Bridgelux Vero Version 2.0 and Cree CXB series), which came out this summer. Some of them (high efficiency bins) are still not easy to get. Some people are achieving close to 70% radiant efficiency with Cree CXB3590 (only 30% of electricity is wasted as heat, and 70% is converted to light energy). A cheaper way to grow plants under artificial light!
I posted several other LED related posts here, but COB LEDs are the most promising (cost effective) DIY project for CP, which requires lots of light (similar to MJ).