I have one on mine but I use a modified x carriage which has a mounting point on it. I probably don't need it so much now with my latest metal hotend but it's handy in case my lower fan expires!
here's what I'm talking about..
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www.thingiverse.com]
You will see a square block on both ends with a hole through for mounting. I designed my own fan mount to go with a 40mm fan I had.
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Waitaki 3D Printer