Inside my microwave is a small 3x5" opening which is covered by a 3 x 5"
piece of thin shinny cardboard. At one time, the four corners of the
cardboard fit snugly into four small slots, so that it barely covered the
full opening. Now those four corners have become broken and worn, allowing
the cardboard to slip 1/4" below the top opening. Will this affect the
performance or safety of the microwave? If so, can I replace the cardboard
with a piece of non-shinny cardboard?

Thanks.
digger