Have you tried to produce lichtenberg figures with the acrylic slab at very low temperatures. I expect it will reduce the velocity of the spark across the material. Be careful it may explode if the material becomes brittle.
Applying the voltage to the acrylic slab after being cooled in a freezer and ultimately to the temperature of liquid N2, I do not know if the polymer can stand this cooling, it may allow slow motion capture of the moving sparks. It's a guess. As it's a chemical reaction it should slow down.!!!
Thanks for your attention.