I see lots of folks here asking about adding ducting to their miners. And while you should really house your miners in a dedicated space with adequate airflow, sometimes we have to make do. So I thought I'd pass on a few things I learned.
1st be careful of the 3-D printed stuff on eBay, some are OK, but others are overpriced and not very well thought out. Not to mention printed out at very coarse resolutions.
Personally I feel that to provide the least amount of restriction you should use a 6" adapter and 6" ducting. I tried a 4" eBay duct adapter on my S9 and the temps skyrocketed *without* any ducting. Then I tried a 5" and it was better, but I could tell temps were still a few degrees better. Finally I had a buddy make a nice 6" adapter, this had no effect on the miner's temps.
Here's a view of 3 miners with the 3 different sizes in ascending order from left to right.
Here's a couple of closer pics of the 6" adapter my friend made and what I plan to use on all my miners.
Here's a mock up of the 6" ducting (I need to clean everything up
), the other one in the background is 5" ducting. I wanted to make sure the 6" worked out before my friend made one for each miner. Also the two blank holes are for my 2 M3s that'll be here mid-late Feb.
Here's a couple of pics of the backing board with the 6" starter ring holes, it is going in a hole cut for an old window A/C unit. I cut this out on a CNC router.
EDIT:
My friend will be adding the duct adapter to Thingverse, so just search for Antminer. It should be there soon.
Here's my parts list:
6" semi-rigid duct
http://amzn.to/2DyBDW5 semi-rigid is kind of a PITA to work with, but I feel it has a smoother tract with less airflow interference.
6" Starter ring
https://www.homedepot.com/p/6-in-Starting-Collar-Take-Off-Snap-Together-SCF6/100396966Duct sealant, used this to seal the starter rings to the plywood
http://amzn.to/2DyvbhT 
