I created a 6 ft x 8 ft walking pantry, but closing in an unused space from an adjoining room. Its on the first floor of a 2 story house, but has an exterior wall and I am concerned about it getting hot and stuffy in there.

While several bedroom closets and the closet under the stairs do not have ventilation, I'm thinking I need some ventilation for the space besides the opening and closing of the door plus gap below the door. Help with temperature as well as food smells.

The ceiling above the space has an 8" flexible air duct to another room, but that is running the ceiling/floor joists so space is limited. Looking for options and thoughts.

One AC guy suggested cutting a 4: hole in the flex duct and installing a 4: register. Sounds simple enough since the space is limited and the 4" register will tie-in without the need for additional duct work. But this sounds like a problem waiting to happen. Especially trying to seal the side of the flex duct to the vent.

You don't just cut a hole in the side of an 8" flex and tape on some 4" flex to a diffuser. You can buy a 8x4 tee or wye that is sheet metal, split the 8" duct and add in the 4". Simple enough as long as you account for the loss in pressure to everything down the line from your wye.