40mm fan booster
thingiverse
This is a pressure booster for a 40mm fan. I don't know the exact angle the fins should be so it might not be the most efficient but it definitely is an improvement.
Download Model from thingiverse
With this file you will be able to print 40mm fan booster with your 3D printer. Click on the button and save the file on your computer to work, edit or customize your design. You can also find more 3D designs for printers on 40mm fan booster.