Morvat Adjustable Spring Access Panel 6"x6" Inches, for Drywall, Ceiling, Electrical, Plumbing Heavy-Duty & Easy Install, Wall Hole Cover Plate, White | Fits 5.5"x6" - 7"x7" Inches | (1-Pack)

If you want plain, they make plastic covers that have spring loaded wires on the back that hold it in the hole yet allow it to be removed. Adjustable Spring Access Panel 6"x6" Inches - Amazon Otherwise a simple small door over it ould work. Either way, you need access.