Plexi plate insert.  These are for insert don't have a lip and sit on a ledge or holders inside the hole.
 
Select the correct size of aperture for your cabinet.
 
1939 (Nova fitting) - aperture size 545 x 300 mm - £74.95
1942 (2012,2032,2052,2062,2072) - aperture size 615 x 330 mm - £74.95
1952 (Hideaway Mk 2) - aperture size 625  x 339 mm - £74.95
1972 (New Cub + fitting) - aperture size 480 x 285 mm - £74.95
1992 ( New Gemini fitting) - aperture size 530 x 285 mm - £74.95