'The Mirror's New'. Please explain

I've just finished watching The Mirror's New, and for the life of me I can't figure out why the glass mirror has been replaced with plastic??

I DO understand that Bierce was foiled in his attempts to dispose of the murdered body, but where exactly is that place he stored it? At first I thought it was a closet door with an embedded mirror. But upon closer examination there appears to be no door handle.

So is it a wall mirror? With a much bigger than needed frame? And the glass was removed (broken) to stuff the body into the wall? And Bierce just happened to have a roll of reflective plastic lying around his bachelor pad? Is that what they are intimating? As if a body could fit in the expected lack of depth of such a space. That's the part I don't understand. How could the interior of the wall accommodate a body?

Any ideas better then this conjecture?