Engraved surface is recessed and just over 1" diameter… 
That’s why masking won’t work either. Unless I could cut circles of masking, drop those in place, engrave, and use regular acrylic paint.
Hmmm…
Just lay a sheet on the bed (with magnets) and have tabs to hold the circles.
See, sometimes just talking thru a problem can lead to an unexpected solution! 
(… and now I don’t have to buy something just for one project!)