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Consider an in-floor return air duct that is 32" x 22". A standard air filter that closest fits the register is 30" x 20".

What materials should I use to frame the duct so it will snugly hold the filter?
Would non-treated lumber cut to size suffice?

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The very idea of a nearly 3' by 2' grate in the floor is troubling... are you certain that this grate is up to the potential weight load that will be on it? – The Evil Greebo Sep 22 '11 at 13:39
Yes, the house was built in 1951 and the grate in the floor is quite durable, heavy, and is dense with metal lattice work. The grate appears to be as old as the house. – zundarz Sep 22 '11 at 14:09
up vote 4 down vote accepted

Non PT wood will be fine.

I would build a 32x22" frame out of 2x4's but I would put a 3/4" square notch around the inside diameter of the frame and set the filter into that notch. That will give you a nice seal for the filter but prevent it from falling through. If necessary, add one or two cross braces across the inside of the frame cut to the depth of the notch so the filter has extra support. enter image description here

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Do you have something to screw into on the sides? I'm assuming so since joists are typically 19.2" OC.

I don't see any issue with wood since some builders will run the return through wall cavities without a duct. You may want to include some notches on the sides (but not under, you still want an airtight fit) to get your fingers around the filter to remove it.

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