KASTLITE Egg Crate Panel 2-Pack in Black | Polystyrene | 23 5/8” x 47 5/8” with 1/2" x 1/2" x 3/8" Cell Size | Manufactured by American Louver | Ceiling Light Covers, HVAC Grilles, Aquarium Eggcrates

If this is for ventilation and can't be blocked - get some black egg crate light panels. Its just plastic grids make of stiff plastic. Easy to cut into the shape you need with some strong snips. Use some cardboard first, trim it down until you get the exact silhouette of the gap, then trace and snip the panel into that shape. Then put the panel in the gap, looks like it should settle in firmly if you trim it right. https://www.amazon.com/KastLite-Polystyrene-Nominal-Manufactured-American/dp/B08L6RXD8L?th=1 Just to show you on amazon what they look like, don't buy there its crazy expensive, but you can find this stuff cheap at any hardware store like Home Depot or Lowes. Edit: another link to show example https://www.homedepot.com/p/PLASKOLITE-4-ft-x-2-ft-Suspended-Egg-Crate-Light-Ceiling-Panel-1199233A/202025149