need advice on wooden box construction

I want to build a wooden box of size about 16 inch height x 24 wide x 20 long. It is a return air plenum. The "box" will not have a bottom (where the return air duct is) and no front (where the grill cover is). Right now the entire wall cavity is the plenum but I need to clean it up. If you want to know the details you can read my previous post.
Should I use 2x2 or L brackets to screw the wood panels together? L-brackets seems easier because I won't have to drive long screws through a 2x2 inside a small wall cavity. But I also feel using L brackets is not as good as a 2x2.
Also, what thickness wood panels should I use? 1/4 inch plywood seems strong enough. However they tend to warp a little over time. If the box is held together with L brackets (say two brackets per edge), warping could create unwanted gaps. 1/2 inch plywood would stay straight, but seems overkill. I see some material call hardboard that is 1/4 inch think with a vinyl coating. Could those be used?
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studs, then liquid nails to hold and seal the ply to the 1X2 cleats.
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On 10/27/2013 08:13 PM, bob wrote:

A wooden plenum would possibly not meet the building code. IIRC you were given proper advice the first time you posted so you should take it. When in doubt, err on the side of caution.
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On 10/27/2013 9:13 PM, bob wrote:

Most plenum I've seen, have been sheet metal. What's the appeal of wood? Sounds like you're doing it for cosmetic reasons? If that's the case, use what the nearby building uses, so it looks similar.
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Stormin Mormon wrote:

Apparently, it is just a box for a return duct, not a supply duct. He posted about this on 10/16/2013 with the subject heading, "isolating furnace intake".
My vote would be 1x2's, 1/4-inch or 3/8-inch plywood, nail or screw it together, use some construction adhesive to help seal it (maybe Loctite Power Grab, which works great). Then use screws to attach it in place. Have fun. It will be hidden behind the wall and doesn't need to be fancy.
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