Nice... also if you had it blow IN, it would suck from only one side...... blowing out collects from the outside on 3 sides... so probably the best choice.

Filter on the bottom is a great idea. I have something similar and I have triple filtration. I used an old squirrel cage blower from my old AC air handling unit as a fan. I put a couple of 4" deep HEPA filter cartridges inside the box around the blower inlet. On the sides, I doubled the furnace-style filters. The inner furnace filters are something like MERV-13 (similar to what you've got) but sandwiched on top/in front of those are cheap non-pleated fiber filters. They do all the pre-filtering of the air, catch most of the big stuff and can be easily blown out and/or replaced when need relatively inexpensively. The blower moves a lot of air. It runs over the air mass in my 2-car garage in a matter of minutes. I ended up building a deflector for the outlet because it was blowing things off the bench across the shop!

I am currently working on a unit similar to yours. I am placing the fan on 1 side blowing out so I can use it as a cooling fan and use the top as an additional work surface. I am using 20” and 14” filters stacked on the 3 sides so the finished top is at 36” to match all the other work surfaces in my garage. Using aluminum Channel to slide the filters in from the top on the inside of the frame. The top will hinge on one side to open to change the filters.

3:16 I was going to suggest adding a filter to the underside. I would definitely have done that, and to provide a bit more distance to the floor, would have raised the base another three or four inches to provide more clearance. And I absolutely would have added a filter to the top, for a total of five filters. As for horizontal space for tools, I would have simply added a solid platform(s) a few inches above the filter on top, comprised of even two or three shelves if need be. Whatever works for you. Just some suggestions. Personally I have no interest in spending even the minimal amount of time to build something like this, as you and others have. Secondly, I don't need something else on my shop floor getting in the way. For general air filtration, I simply tape five filters to a box fan and suspend it above the fray, and use wireless on/off. The speed knob is on the bottom so it can be easily changed; I may modify that and extend the wiring so I don't need to step up to it. For filtration close to where I'm working (besides tool ports/vacs/etc.), I place one (or two) box fans with a single filter taped to each, next to where I'm working, e.g., if I'm routing long boards to make molding, I'll place the filter fans on my work table. They collect a lot extraneous dust before it ends up airborne.

I like your design. I was just letting the 'ol brain kick in and I think you might have a great setup for a down draft table. Your table top would provide a nice area to allow a top with holes drilled in it to make a table top sanding platform. Just a thought. I have been watching many DIYs for how to do a whole shop air filtration system and stumbled onto your channel. I like what I have seen you do. My shop is 3200 sq ft and I have a lot of machines creating dust that the dust collection just doesn't pick up. I'm looking to build filtration system that hangs from the ceiling and can move air from the entire area and do it efficiently and effectively. These four or five filters look like the way to go. I'm trying to figure out how to get air flow coming from both ends of the shop with the use of one fan. Not sure that is possible. I might have to put in two systems, but one would sure be better than none at all. Thx for your video.