Here it is in action...
I made a plate and tube out of copper to mount the fan on my CharGriller Kamado Kooker... The 3/4" copper pipe is hammered square to fit inside the blower opening, it fits really well, everything is soldered together for a tight seal......
I drilled a hole in the copper plate the size of the copper tube then soldered it in place using a torch, solder and lots of flux (like you would when installing a water heater).
Did you use a flange?