For a removable grille opening, if you can find something behind it that is solid to attach a toggle bolt or a tumble toggle behind, or to run the folding end behind, you could drill some holes and use a washer under the head of the toggle bolt
Lexan or Coroplast could work, and you would just have to have something behind the opening that the folding part of the toggle would go behind. And when you remove it, be able to catch the toggle or at least find where it falls.