This is a remix of
https://www.thingiverse.com/thing:1830918
Which was the best Duet box I could find and was one of the very few that put the cooling fan in the right general area (blowing on back of the controller). It also used an 80mm fan of which I had a few handy.

Decided I needed to do a few minor tweaks to get it to more my liking though.

I replaced the grill cutout on the cover with some Duet branding
Thinned up the exterior wall of the enclosure a bit. Changed the outside dims on the cover as well.
Removed the inner walls that the Duet installed on to and replaced with just some screw bosses. I did this both to save plastic and free up airflow.