https://www.amazon.com/dp/B0751B1P78?psc=1&ref=ppx_yo2ov_dt_b_product_details
Rear (Aux.) Enclosure Fan HEPA Filter Holder

I could not find a HEPA filter holder for my Creality K1 that was easy to print and install so I designed one. It uses a stock replacement Vacuum Cleaner HEPA filter that is easy to source online.

The holder bolts to the fan using 3 x 25 mm Socket Head Screws. (The original screws are just a bit too short to reuse.) Once the bottom is attached, the filter can be inserted into the top and the top will snap into place. It's a snug fit but it works perfectly. There are thin retaining lips on the two vertical tabs to hold the top and filter in place.

This was printed in PLA+ but is probably better in ASA for temperature resistance. It printed in 1 hour - 3 minutes.

Temps: 220/50

Filament used: 50g

No Brim

Normal Supports on Bottom Part Only.

Speed: Outer wall 200 mm/s, Inner walls 300 mm/s

15% Infill @ 250mm/s

HEPA Filter: HEPA Vacuum Filter for Kenmore EF2 (EF-2) or equal.

4.3"L x 0.75"W x 2.85"H

https://www.amazon.com/dp/B0751B1P78?psc=1&ref=ppx_yo2ov_dt_b_product_details

Assembly

Remove the four screws holding the Rear Enclosure Fan in place.
Place the Holder Bottom over the fan.
Insert replacement screws through the Bottom and tighten the screws.
Insert the HEPA filter into the top (Air Flow direction does NOT matter. The filter works equally well in either direction.)
Snap the Top and filter in place. Done.

K1 Rear Fan Internal HEPA Filter Holder

0.2mm layer, 2 walls, 15% infill