I sourced a 4u server case with the same dimensions from Craigslist for dirt cheap and with some time and DIY I was able to get a total of 9 GTX 1060 cards in the case running stable at 22.8 MH/s @ 85w and staying at 70C each with the kid sealed..
I can get 8 of the 9 cards to run at 65C but card number 5 won't go below 68C, I'm pretty sure it's due to the spacing of my fans as it is the card with the fan against the side of the case.
Here is a video.
https://youtu.be/S_A3X-OsCkwVery clever work. I was impressed by your support bracket for the 2nd HP PSU and the PCIe expansion board which makes up for the blocked slot. The one problem that I see is that bigger cards would push too far back to allow for the same orientation of the HP PSU. But with smaller cards that's a nice setup.
What did you make the fan brackets out of? I don't recognize what you are using and was wondering whether it's custom, or repurposed from something else.
I would try Delta fans if that is not what is shown in the video.