I need a battery box vent for my solar batteries enclosure. Many people sell fans that can be installed in PVC piping, but they are expensive.
I have a bunch assorted fans from 2" to 4" from the MIT swap meet, so I set out to make a ducted fan assembly for them.
I found that the white PVC reducers were just the right size, so I took some white expanded PVC foam board, and using a 4" hole saw bade a simple flange. I glued the flange to the reducer, and used some long 6-32 screws and now have a nice ducted fan that can be inserted into the 1.5" PVC vent line. Made another for a 2.5" fan.