Return Air Cabinet With Slight Corner Variation

This particular return air box is almost identical to other three piece boxes I make with the exception of the way the 4 corners are notched for overlap. This style, when coupled with a series of two pop rivets in each corner eliminate the need for the corner bracket reinforcements seen in others on this channel. Most air handlers for residential and light commercial applications weigh 80 pounds (36kg) or less so a properly made 26 gauge return air plenum box is more than adequate to support them properly.
With the addition of either a wide bead of silicone caulk or 2'' wide Armaflex tape on the flanged surface of the plenum, the return air connection into the bottom of the air handler is pretty much air tight. Now there will be a small amount of leakage around the actual return air fittings flange attached to the side of the return air box....but it is only minimal. Another thing if silicone is used for the seal between the furnace and the box, it can get a bit messy working alone. It's best done when there is a helper on the job that allows you to set the furnace directly down perfectly straight on the flange.
As said frequently in past videos, there is not one 'correct' way to fabricate a return plenum box. There are as many variations as there are fabricators in the field. Some are good, some are real good, some are bad and some are real bad. My suggestion is to not be the guy whose name is associated with the ones referred to as 'real bad'.
A video highlighting a return air 90* elbow similar to the one shown at the end of the video can be seen here:    • Return Air 90 Degree Fitting- Integra...   On a new construction project or a project involving an upgrade in the total return air drop will have this or a similar fitting installed with it. There is not much more frustrating to a home owner wanting to change his own filter than a system whose filter is all but impossible to do easily. It's good practice to walk away from a job that not only functions correctly but is aesthetically appealing to the people that now own your name on their project. In a small town your name will seldom be mentioned concerning a good job, but it will be repeated over and over about a bad job. However the good job will morph into so much additional work through recommendations than you can imagine. That's just the way small town communities work....
So don't necessarily duplicate what you see in the video, but if it'll help you improve what you are currently doing, have at it. What good is knowledge or technique if we take it to our grave without sharing......