This is a simple remix of Midevil1's fab 120mm fan base. It simply has the addition of small dovetail locking pieces to ensure they cannot come apart. The dovetail recess is 0.2mm larger diameter than the dovetail so this should account for printing tolerances.
The image of the actual item shown above is taken from Midevil1's original listing. Full credit for the original design :)
Edit 250318 - I have replaced the files with a v2. The original had extremely tight tolerance on the dovetail parts which, depending on printer tolerances and filament type, made them very tight to join (sanding or lubrication may have been required). The tolerance has been increased to 0.6mm which should make this one much easier to slide together. If you have printed the earlier version they will work but may require a slight sanding of the spigots and use of washing up liquid or cooking oil to lubricate.
I have also added a test piece which I highly advise you print first to check the tolerances before embarking on a 24hr print.