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I made a very similar unit a couple of years ago. I used a blower fan given to me for free by my air conditioning people who get usable fans regularly when replacing CA/CH units. I then made a slope to a verticle cavity separated from the blower intake by 2 filter slots. I use a cheap filter to catach the big stuff and a hi-efficiency filter behind it to catch the small stuff. It does blow a lot of air into the shop but seems to filter very nicely. Putting power outlets on the exhaust unit is really nice as it allows one to plug in sanders and tools at the unit.