Is this is a wood door with 1/4" flat panels?. If it is you can cut off the interior stops that hold the panels in. Measure the opening and deduct an 1/8" or so from the width and height. Have the glass cut locally. Buy some small molding that is similar in size to the stops you removed. Apply some window glazing to the exterior stops. Insert a glass panel in the opening and gently press it into the glazing compound. Hold the glass in place with those little metal glass holders( forget the name). Local hardware store should have them. Cut the molding and carefully tack it in place. Clean off the excess compound from the exterior. Paint the exterior stops and just cover the glass about a 1/16- 1/8" to seal it from water.
Ron