This is the bracket I designed to work with an inline fan.  I mounted it on each side of corrugated plastic and have used it for over a year no issues. I printed mine in PLA, it has weathered well.  It would not hurt to print in PETG though if it is going to get super hot.  Mine is in a basement window so the air is always cooler.

I use M3 screws to clamp the 2 brackets together on either side of the plastic sheet.  This provides plenty of hold.  The screw length will depend on the material used. 

The inline fan will slide directly into the collar.  Connect the 4 in hose to the other side of the inline fan using a screw band.