This is one overbuilt Corsi-Rosenthal air filter box

After a bunch of design and planning out the cuts, I had most of a Sunday to work on cutting lumber and assembling my Corsi-Rosenthal box. I started with the miter saw and the 2x4s since that was the least-familiar material to me. I did a little bit of planning to try to minimize waste on the 8-foot lengths of lumber and feel like I did pretty well. I only had a few 1-2 in blocks and one ~13-inch length left over.

Photo of workbench centered on miter saw with 2x4 prepared to be cut; some finished 2x4 cuts lie to the left of the saw

Since the 2x4s were inexpensive and intended to be studs in a wall, the finish was not especially nice. I sanded the cut pieces with my electric sander and 80-grit sandpaper, which was enough to make handling the pieces quite a bit more comfortable.

Photo of finished 2x4 cuts organized in groups of 4 lengths, each with 4 lengths for a total of 16 pieces; the lengths of each piece are lightly penciled on the edges

The next cuts I needed were for the top and bottom sheets. I thought I had some 5/8-inch plywood that would work, but it was too narrow for my design, so I had to use 1/2-inch particle board that was a shelf in its former life. This ended up being somewhat difficult because I still haven’t developed enough skill with my circular saw to execute a decently long cut. The first 27-inch cut was square enough, but I ended up flexing my guide when making the 23-inch cut and lost almost an inch on the cut. Fortunately, my design had generous tolerances and the off-square plates didn’t cause a problem.

Photo of particle board laying across two workbenches with a gap to allow the circular saw to cut the board; a metal rail is clamped to the board to guide the saw

I had the most fun assembling the pieces. Marking out the screw holes and drilling was satisfying, as was seeing the box take shape. I did chew up my fingertips switching my battery pack between my drill and impact driver for drilling and driving 76 3-inch screws.

Closeup photo of 2x4 with pencil markings for each hole, with drill in frame

Photo of assembled box without top or bottom on top of workbench; many tools are around and sitting within the box

I screwed on the top and bottom sheets before I cut the 16-inch fan hole. Since the 2x4 structure was not perfectly flat, I used some cardboard as a compressed gasket between the 2x4 frame and the sheets. This should be adequate for the airflow I’m expecting–and if not, I can always tape or fill the seams.

I got some photos in the afternoon light to compare against my render, and then built a crude circle jig for my jigsaw. I have a big box of scrap wood and I found a board that was very roughly suited to the task. I used the jigsaw to cut a slot in one end, measured the radius of the circle (8 inches) and pounded a finishing nail through the board into the center point of the top sheet. I drilled a starter hole to get the jigsaw blade into place, and then slowly rotated the saw around using the crude jig.

Photo of assembled box without a hole cut

Photo of crude jig sitting on top of successfully-cut hole

Honestly, I’m surprised it worked so well. I’d seen some “5-minute fix” style Instagram videos doing the same and doubted the method, but it did mostly work. I didn’t support the loose circle and it did pinch the saw blade toward the end of the cut, but I was able to reposition it and finish it without compromising the circle.

I brought the box back into my garage and set the fan over the hole to get an idea of fit and I was extremely pleased.

Photo of fan sitting on top of assembled box

After being on my feet for most of the day and pushing my knee beyond where I should have pushed it, I left the finishing to another day. Since the filters are a nominal 20 inches on the short edge, but actually 19 5/8 inches, I need some finishing hardware. I think I’ll probably 3D print some simple clips, and maybe use some drawer liner material to frame where the filters will sit.