Model originally uploaded to Thingiverse at https://www.thingiverse.com/thing:3643870.
I have a bunch of 4020 blowers and found they're quieter than the 4010 fans I have. Plus they tend to flow better when there's a restriction (blowers maintain pressure better than fans). I couldn't find an adaptor on thingiverse, so I made one.
The part where the blower attaches is from another design I'm planning to upload in the future. The blower slides neatly in place with the mounting tab within a recess and the guides on the opposite side on the inside of the blower. Those guides keep the blower from wiggling (since it's attached by only one screw) and reduces the air leakage from the high pressure side back to the low pressure side of the squirrel cage (at least for the small portion it covers).
If it's noisy when you assemble it, please be sure you don't have over-extrusion on the guides going into the blower. Sand them down if you do to make it smooth and provide clearance. Please also be sure that the blower is pressed onto the adaptor completely.
I uploaded two versions, one with a recess for the head of a 3mm socket head cap screw, the other with a recess for a 3mm nut (in case you want the screw coming through from the other side, this is handy if you have button-head cap screws). Either will allow you to use a 20mm screw. The one in the picture doesn't have the recess, so I had to use a 25mm screw (that's why it's sticking out so far on the other side). I didn't feel like reprinting it after I added the recesses.
Edit: I had the fan mounted with a horizontal axis at first, but rotated it so it had a (near) vertical axis with the intake on top and it seems to have reduced the noise in the room even further. I've uploaded more pictures showing it in both orientations on an Ender 3.