PimpRig have posted a How-To article, which shows you how to add a custom window etch to your case:

    There are many ways to etch plexiglass. You can use a chemical (usually acid of some sort), you can also use a roto tool to engrave into the plexiglass (I feel this gives an uneven finish, unless that's the look you want on your design), and then there is the forced method... using air pressure to force grit medium to sand or etch the exposed surface.

    This is just one of many ways to achieve an even etched finish on a sheet of Plexiglass.