I wanted to add a cooling fan to the outside of the base of my FolgerTech Kossel 2020. A 50mm fan is a good fit for that as it aligns with the extrusion slots for easy mounting. The sides of the base is made of 2 2020 elements with a 20mm vertical displacement.
The only problem with mounting the fan directly to the extrusions is that most of the fan output is blowing straight at the upper and lower 2020 element.
So I created a funnel-like adapter for a 50mm fan that would collect the air and then direct it in between the 2020 extrusions, all with the least amount of resistance possible.
The rectangular outlet is 19mm tall to allow for quick draft-type printing. With an 8mm deep fan one needs M3x45mm machine screws to mount the fan with adapter to the extrusions. 2 diagonal screws are sufficient.
Infill is only used for a few layers on the base and should not matter much or help with printing speed if reduced/removed. The rest is almost completely solid shells and took close to an hour to finish for me.