Here's remixed duct for Anycubic i3 Mega. Uses standard 5015 blower fan. Clips of duct will be tight, wiggle duct while pushing on bearings.
As requested added mirrored version of cooling duct. Added also adapter for 40mm fan, design may not full fill flow technology standards, but 40mm fan can be attached :). I do recommend to use radial fan as they produce greater pressure.
Remix information will be updated later.
Edit: added mirrored version. Better to use this on stock i3 Mega.
Edit: added adapter for 40mm fan.
Edit: added remix information, remixed of https://www.thingiverse.com/thing:2803399
If your slicer supports variable settings, you can adjust higher infill and number of shells on area of the clips. If you can't, then use 4 shell and 70% infill. ABS recommended as PLA will get soft when using duct.