Updated to version 2, with correct diagonal airflow. The original was blowing almost completely horizontal.
Rear 40mm fan to blower duct for the i3 MEGA Ultrabase, and suggestion for replacement of noisy fans in the base.
I used 2 Scythe Mini Kaze SY124010L for the stepper drivers. Just use the original fan screws, and the original fan plug and included adapters to make a Y-adapter. The Scythe SY602012L will fit the PSU. The SY124010L will also fit the hotend blower, so 3 4010s. The rear blower is using a 4015 size for more air since the Scythe is only 14 CFM, but it might actually work fine.
Update 11/25: Added optional side cover and duct. Uses the original screws, but puts them on the inside. The nuts can be used for the rear blower. It needs longer screws, like M3x7 or 8.
50x15 v2: The fan sticks out on the right side some, so it can't fully travel to the right of the bed, unless the mount is cut off. It's about 8 mm from the edge.