I saw this earlier today - I love the design. It allows the maximum surface area for cool air over the heatsink fins which should lower gpu temps considerably with much less fan noise and power. Much better than the old design where the fans that cool the GPU pull air from the next card’s super hot backplate. With this design there wouldn’t be a need to have additional fans blowing air from front to back of the cards as long as the room has good air circulation normally and the heat doesn’t accumulate in the room as a whole.
I’m not sure how he secured his GPUs to the backboard, but I would at least zip tie the pcie mount to the backboard to secure them and use either motherboard standoffs or some other method to secure the risers other than glue because that melts.
My only other concern would be using wood as a backboard against hot backplates. This concern could be alleviated by drilling some breather holes through it where the GPU contacts the wood to allow for better heat dissipation of the backplate, by using aluminum or some other material that disperses heat more rapidly, or by making sure the GPU doesn’t actually contact the wood backboard directly so it has an air gap.
The issue some mentioned with the PSU power cables reaching the lower cards might be addressed by mounting the PSUs on the sides in the middle of the rig with the fan intake facing out unless your cables are long enough or you have extensions. That’s pretty hardware specific since PSUs are widely variable in the cables they include.
good luck, looks like a great project!