You could also stick self-adhesive reflective film (chrome finish) to the whole of the perspex cover; then, with a sharp scalpel/other blade, cut around the middle portion of the film; subsequent removal of the centre section will leave a ring of reflective film on the perimter of the perspex cover.
One-way mirror film would work, for example:
http://www.visionwindowfilms.co.uk/O...FVDLtAodZwIAKw