Mini-CR made from two IKEA Starkvind filters.

Cost for me: £51 (USD $61) and an hour of experimental assembly.

Ikea parts;

• Two STARKVIND filters - https://www.ikea.com/gb/en/p/starkvind-filter-for-particle-removal-30461943.- £11 each
• Two SIBBHULT brackets: https://www.ikea.com/gb/en/p/sibbhult-bracket-white-10417734/ - £1 each

From AliExpress:

• Three-fan hydraulic bearing assembly with speed variation: https://www.aliexpress.com/item/1005003180338181.html - £26.91 (inc tax and free shipping)

Also some wood - spare/recycled bits and pieces for me.

By sheer co-incidence the three-fan assembly was a near perfect fit for the long edge of the STARKVIND filter. I put a frame around the filter first, then did a bunch of ruler-less measurements of the filters dimensions and where to saw the 10 mm wooden planks. Then I assembled the lower parts with the shelf brackets, and finally drilled and screwed the fan assembly to the lower piece.

Everything was fine to that point, but the two filters are different sizes by 3mm in ne direction. I'd only measured one filter of course. So i hat to get use a utility knife to remove enough wood to allow the bigger filter to go in without buckling.

Operating noise

Sound meter positioned at 1 meter away at floor level

This Mini-CR

36.5 dB at lowest variable fan setting
46.5 dB at mid variable fan setting
51 dB  at highest setting.

A Levoit 600S for comparison

42 dB - setting 1
46 dB - setting 2
52 dB - setting 3
56 dB - setting 4

Filter efficiency:

The IKEA filters are E12 rated, which is not quite as good as HEPA 13

^ from https://en.wikipedia.org/wiki/HEPA

I have not yet determined Clean-Air-Delivery-Rate (CADR) for this box