Why would you want to go through all the trouble of building this?
GPU rigs are fine with open air. I mine in a tropical country where ambient temp is higher than average. I have a small 39-GPU rig farm and run them at around 65 degrees using simple fucking fans. None of my GPUs have died. Been mining for a year now. ROI is real.
How do you vent outside?
You won't be venting enough heat out of your attic. You'll need at least 6,000 cfms coming out of your attic and (2) 6 inch vents won't cut it.