There is a vinyl product (adhesive backed) called etchmark. It simulates etched glass. The idea being, you cut out whatever letters, logos, shapes you want and stick it onto glass (or plex) to give the glass an etched appearance.
This vinyl is 2 mil. thick, so it will create a thin airspace between the two panels.
Alternatively, you get get a reverse cut decal (mask) and sandblast one of the panels instead. I'd say sand it, but it may be difficult to get a crisp edge up against the mask with sandpaper alone. THis effect might get you an edge lighting affect that could be good or bad depending on what you are aiming for. However, without even lighting all around the panel, it might be 'sporatic' rather than evenly lit.