Comment by breput

You'll definitely want to bump thickness up to 2.0 mm for more rigidity. Otherwise just measure the diameter of your filter and maybe round up slightly.

I put a layer of electric tape around the flange where the filter adapter inserts into the filter and it makes a very nice airtight fit. Finally, just print it with the big end facing down and you shouldn't need any supports.

screwhole=5; // fan screw hole diameter (5)

filterhole=59; // HEPA filter hole diameter

thickness=2; // Thickness of plastic layer (1.5)

insert=10; // Length of insert into filter (10)