There you go. This is the stable configuration with 4 cards. There is a bit of space between the cards and the PSU, so I'm currently fitting a fifth one in there 
It's awesome!!
Where did you get the hardware for that open air rig?
Thanks! I actually designed it myself
. The aluminum profiles were ordered cut to the required lengths (4,50€/m). Including nuts, bolts, angular connectors and the black stopper caps, it was about 70€.