I built a heat recovery ventilator (HRV) from only sheets of corrugated plastic and adhesive.
EDIT: SEE PINNED COMMENT for some corrected efficiency metrics. To summarize, my HRV is actually 60% efficient (which is still decent!), not 85%.

Even so, it works great! My HRV is powerful enough to deliver fresh air to most of my house, and it’s designed to use 4” thick MERV13 filters to help keep harmful particulate (PM 2.5 and 10) out. It greatly reduces VOCs and CO2 as well. My indoor air quality has significantly improved. The cherry on top: it cost me less than $300 in materials to build!

Intro 0:00
What HRVs do 0:32
The drawbacks of other DIY HRVs I've seen 1:18
My HRV (THE REVEAL) 2:58
Its performance data 5:12
The cost of building it 7:25
The drawbacks of my design 7:50

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My video about why I now use an ERV instead of this DIY HRV:    • HRVs vs. ERVs: Why I Ditched My DIY H...  
My DIY ERV installation (stepbystep build):    • DIY Energy Recovery Ventilator Instal...  

Designed by Instinct’s video (this one is most helpful for actually building):    • DIY Heat Recovery Ventilator  Full B...  
AlexGs Aquarium’s video:    • DIY HRV (Heat Recovery Ventilation)  

The products I used for quantitative testing:
CO2 meter: GasLab Plus CM501
Particle counter: Dylos DC1100 PRO
VOC meter: Forensics Detectors FD90AVOC
Hot wire anemometer: Testo 405i

Necessary tools and materials (should cost $200300 total):
Pen or pencil
4 mm coroplast sheets
48" drywall Tsquare (for measuring)
Utility knife
XACTO knife
Hot glue gun (Adtech Pro80)
Hot glue (Adtech full size multitemp sticks)
Filter: 2 Filterbuy 14×14×4 MERV 13 filters
Fan: for the design in this video, I used the Terrabloom 6” EC inline duct fan. HOWEVER, if I were to build this again, I’d actually use the AC Infinity Cloudline Pro S6 because it has a higher static pressure and delivers more airflow, more energy efficiency, and is quieter. It is however, a longer fan so may be harder to install.

A few of you have requested CFM measurements for my HRV, so I’ve gone ahead and taken them! So, I measured the CFM at the supply outlet at various fan speeds. Here’s the data:

With an anemometer distance of 68 inches from the outlet:
speed 1 of 8: ~100 cfm
speed 3 of 8: ~150 cfm
speed 6 of 8: ~250 cfm
speed 8 of 8: ~330 cfm