Supply air gap between coil housing and plenum

I’ve been sealing a lot of gaps and holes in my central unit. I need ideas for one gap from some of you smarter and more experienced people than me.

There is a significant gap between the coil housing and the plenum. The plenum is beyond the coil housing by a good 1/4" to 3/8" across the back side of the unit. Not much space to work back there but I would like to stop losing so much air to the basement from it. Any ideas?

Thank you in advance.

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Is a photo out of the question?

An A/C unit, right? My first thought is a somewhat rigid, somewhat flexible foam insulating material. Some comes with an adhesive strip you peel the wax-backed tape off of. Usually I stroll into a supplier of commercial A/C equipment and play dumb, which I do pretty good. They usually have something that works.

Peel the tape and jam it in the crack, then add duct tape as needed. (Will about a 1” backer rod work? It won’t have the adhesive strip, but there’s always duct tape.)

Second best is finding the “old guy” at your Home Depot or Lowe’s and groveling. I’m good at that too.

My BIL plugged it with a piece of Corning purple foam board. I was worried about the temperature limit of the foam board. That may be an unfounded worry. Its upper limit is 165 F and the max discharge temp of the furnace is 185 F. I don’t believe furnace heat exchangers get that hot and the gap is at the top of the A/C coil housing, which is 17" away.

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