Testor's Dull Cote (flat) or Clear Cote (gloss), spray lacquer, available for a couple bucks at any decently stocked hobby or craft store.
Spray from about foot or so away, light coat, let air dry in dust free environment.
Do cover the lenses. Dullcote will definitely fog clear parts.
DO test on something else first, as well. Just in case...
This will not provide armor plate protection from handling wear (scrapes, etc) or fading due to constant sunlight exposure though.
If you want to be certain of preserving the autograph, it needs to go behind glass (in a display case, for example) and out of direct sunlight.