The adapter is used to work with the original non-HEPA Filter Cap.
As seen in the pictures you need to cut off the end of the cap at the beginning of the venting slots.
After this you need to glue in the adapter. Since the adapter is NOT symetrical you need be careful to glue the small fins in the top position.
The glue you should use, should be a little gapfilling.
Print it with the inside down and with support, the material don't really matter. I printed it in a cheap PLA and it works now for 2 years, without a fail.
The author hasn't provided the model origin yet.