These look pretty nice! I wouldn't want to stack them though since heat rises
We've done quite extensive temperature measuring on stacks that are 5 rigs high, and we've noticed that the heat dissipates as it moves upwards. The cards at the top of the stack are definitely running several degrees warmer than the cards at the bottom, but there are two ways around that:
1. Reduce the ambient temperature of the room. The cards will naturally sit at 73° C - 77° C without any additional airflow; lowering the temperature of the room by 10° C makes any rise (of several degrees) inconsequential.
2. Bolt a box fan at the back of the rigs higher up in the "stack" (or use a free-standing, extendible floor fan) to move the hot air away from the rigs.
Edit: just to reiterate, this isn't some magical solution that removes the need for temperature monitoring and additional fans, it just reduces the box fan requirement for most of the rigs and lets you focus on cooling down those that are running unacceptably hot rather than wasting power on myriads of fans.