Two press-lid paint cans head-to-head using a polymer bolt and spacer at center to hold the lids together, possibly with water in the cans to ballast them enough to float beneath an insulating oil surface (in a plastic container sitting on a balance or digital scale) should show weight change when a high voltage (AC or DC) is applied.
Transformer oils run about 300 to 400 volts per mil dielectric strength. The 15 KV, 50-60 Hz rms (~ 21 KV peak) from a neon sign transformer should be ample. Fred
