I made this air filter yesterday afternoon. Was getting tired of breathing all that sawdust. The blower is from the downdraft of a kitchen range that we replaced a few years ago. Just couldn’t throw it away. It’s 1/8 hp, so it’s at about the point of a commercial filtration system in the 600-800 cfm range. The box was made of scrap plywood I had around. The big expenses were $20 for the coarse filter and another $20 for the fine filter, so I’m in a total of 20 bucks instead of $300 or so. The coarse filter is electrotratic and washable. It’s rated at MERV 8. I used almost all of the 25×20 filter by covering the top and both ends. The fine filter is a Shopvac HEPA filter. I believe it’s rating is 95% at 1 micron and 85% at 0.3 micron. I got really lucky as it slig snugly on the inlet to my blower (6”). The whole box is about 23×14 x 13, and weighs maybe 15 – 20 lbs. I was planning on ceiling mounting it, but it’s very convenient for moving around the shop as it is.