Sprayed-on Dullcote, either from a can or using an airbrush, should work okay, as it usually dries (to the touch, at least) within seconds, especially if airbrush-applied.
Because it’s lacquer-based, don’t apply it with a brush.
I’d guess that you could also find other clear finishes made for use on plastics at any hardware store, many of them acrylics.