Creality K1 filter (remix)

This remix of the original includes an inner ring that effectively makes the internal dimensions both completely square (no rounded corners) and small enough to allw efficient use of these filters: https://www.ebay.co.uk/itm/286812886098 (10x Black Activated Carbon Foam Sponge Air Filter Pad 12.8CM Square).
Having reduced the intenal dimensions, I can get 4 filters from one sheet.