I hunted around the house and finally found an old broken plastic wiring harness retainer from, of all things, my old 1971 Hemicuda Convertible! (So at least there's now some Mopar in this Pontiac...) It was made of the same type of heavy plastic as the grill.
I cut a small section off and filed it to the correct width but left it with an extra amount of material 3/8" long so I could handle more easily while fitting it into the grill space. I also pre-scored a line before the 3/8" tab so I could break it off once it was glued in and set.
