I've just finished an interesting design thats yeilded about 6.7 nf
with I'm guessing 50 Kv strength. Its a stacked plate cap that uses
corrugated cardboard and mineral oil for the dielectric. I place 2
pieces of cardboard between each plate to give about 250 mil (3/4 cm)
spacing. All the pieces of cardboard are oriented so that when looking
down on the stack you can see through the corrugation if that makes
sense. then i place the stack in a container and fill it with oil. The
orientation of the cardboard lets the air bubbles flow up and out quite
nicely. The most expensive component is all the oil and, I believe,
they will have a 'self healing' quality. If theres a puncture, all I
need to do is shake it around to jostle the oil a bit. The dimensions
are about 20cm by 35 cm by 35 cm for, as I said, about 6.7 nf. If you
wanted more farads in less space then you could probably just use one
layer of cardboard but I just needed about 7 nf for my NST resonant
condition and wanted to be sure about the dielectric strength. I
haven't tried this out yet as I'm still building the rest of my coil
but I can't see any serious flaw in my design so far. Good luck and
happy coiling.