Thread by @DavidElfstrom

Just in! Here's the performance of @JenniferKShea's wedge box with a Utilitech-style fan and two 3M Filtrete 1900 MERV-13 20x20x1 on the back. 🇨🇦 Cost: Fan: $28
Designer adhesive roll: $14
2 Filters: $42
Total: 🇨🇦$84 for estimated 335 CADR (519 sqft room)

It performs like Honeywell's largest air purifier, HPA5350BC (🇨🇦$300 sale price/$380 regular). The Honeywell has a greater turndown range but uses more power.
These are updated values for noise and power, measured at the same time as the ones for the DIY wedge: /2

At lower speed settings though the 2-filter wedge design beats the Honeywell in noise, flow, and power due to its large filter area. /3

Making the wedge air cleaner is straightforward. Hinge the two filters on the inside edge. /4

Position over the rear of the fan with the arrows pointing toward the inlet. /5

Trace cardboard top and bottom pieces and cut them out /6

As with previous versions, optionally cut a channel for the power cord /7

Tape a shroud on the front. For this fan, the optimal is a 13.5" opening. (this came from earlier flow measurements). For other fans, can use tissue paper to see where the flow changes direction. This front shroud greatly increases flow through the filter. /8

I've also added an internal shroud of 21cm diameter with tape. This adds about 2% flow and a small noise reduction. Not strictly necessary. /9

The finished version, sans the decorative adhesive shelf liner. You should also use better tape. The painter's tape I used here was for temporary assembly to test. /10

Here is the wedge air cleaner on the test rig mounted sideways because of the feet. A flow capture hood is attached to the outlet. Backpressure compensation feature was used for the measurements and multiple points were taken to obtain an average. /11

See the earlier 1-filter version thread with other details, including frequently asked questions, such as safety. /12

Corrected table, and photo. (original states shroud size as cm, when it's in inches) /13