This morning I am working on a custom cold air filter box. Basically a plenum with a filter rack riveted inside. I fabricated the plenum and just finished installing the filter rack. Here are a few basic before and after pictures.

Here are a couple pictures of the finished plenum without the filter rack installed. As you can see, I have left a slot in the plenum ( 1 1/4" wide ) where the filter will slide in and out.

Next I have to fabricate the 1"x1"x1" channel pieces that will become the finished rack inside the plenum. The plenum is a basic 16" x 24", so I will need two 23 3/4" rails for the long sides and one 15 3/4" rail for the short side. Notice I have subtracted 1/4" from each rail. That is so the rails fit easily inside the plenum without any unneeded force, while allowing for movement in the plenum for easy installation on the job site. I have also cut some 45 degree notches on the 15 3/4" rail so that when the filter slides inside, it won't get caught on a bent corner of metal.

All that is left to do is rivet the sections of rail inside the plenum and the rack is installed. I usually start by installing the 23 1/4" sections first and then slide the 15 3/4" section down inside the other two rails. Now the customer can pick it up and install it on the job site. I have used some basic filter material in the picture to show how the filter would slide inside the plenum. I would not suggest using this as a filter for a homeowner. Buy some high quality filters that will last.