Open Fan Mount for Rambo V1.3 Board - 70mm

My early-era Rostock Max v1 has always run hot in the electronics bay (beneath the heated bed), and during long prints the stepper drivers can sometimes overheat and miss steps, leading to layer shift and spaghetti. I originally had a 10mm fan (from the stock hot end) wedged in there but wanted something... bigger.

P0tluck made a great 40mm fan mount for the RAMBo v1.3 board (https://www.thingiverse.com/thing:2740591), but all I had on hand was a 70mm fan, so I reworked his design to take a