Can’t I use a piece of ½” plexiglass?

Probably, so long as your approving authority (DOT) is not particular about the specs.  The reason is that by the nature of plexiglass manufacturing, it typically “underruns” so the ½” nominal thickness can actually be outside the .5” +/- .005” that is typically specified.  That is why we use 5/8” thk. Plexiglass. You have enough variables in field testing, let’s just take that one out of the picture.