Those capacitors seem awfully large "a foot long by a few inches in diameter" for "40pF at 20kV"! The extended foil is good, but the dielectric may be plastic (probably mylar) which may or may not have low loss at RF. I'd be inclined to buy a few if they're cheap enough, and see how badly they head up in spark discharge service, before I committed to a full scale purchase. The omens are not auspicious, to say the least! Ed Phillips