Watch for overheating.

The reduction in size restricts air speed.

Look into flex vent from Home Depot use bigger like 12 inch put the whole miner in it.

Just about 1 foot inside on intake

 put a small  hole in the vent to allow your pcie cables .

Then make the exhaust side as long as you want .

Just go to Home Depot and search for flex duct 12 inches or 10 inches.