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 MERV-13 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!