Years ago I made an air cleaner from scrap wood, a furnace blower, variable speed switch, and some 18x24" furnace-type filters. The plans came from the web, and I apologize for not remembering the author. The electric motor died recently - aw, shucks. I tried finding a replacement blower motor, but nothing seemed available locally, and Ebay sellers wanted too much for a blower (PLUS the SHIPPING!!! It would have been $125 at least).
Googling around I found what looked like an affordable blower, generally to be used as a carpet drier fan. It is supposed to be available at Home Depot or Lowes. HD accepted my on-line order for pickup. I thought I was set, but the order was cancelled an hour later, no reason given. I then ordered from Amazon a "Lasko Pro Performance Blower Fan #4900" for $60, more than the HD price, but available for speedy delivery.
My home-made box has open sides with rabbeted slide-in filter holders. The whole thing was made for the furnace blower but is now just slightly too narrow. The filters barely fit on one side, and not quite on the other. The total dimensions of my box are 25x15x19" L/W/H. Obviously (see photos), the box could have been shorter and lower, but this is what I have, and using my existing supply of furnace filters is a plus. The extra rectangular opening above the blower outlet was for the electric box with the variable speed switch, now not necessary, because the fan has off, and 3 speed settings. Plus this thing has extra outlets ... Obviously the disadvantage of the current setup is that the switch and outlets are covered up by the filters on that side. Have to find a solution for that. I hope this link does go to the 5 photos ...