Thanks for the help with the equation! Depending on how many bottles I can get, I'll see what I can do. I might even use plexiglas as a plate and layer metal sheets between them, because it is less messy and I have some plexiglas. Will it thermal shock? I might submerge it in oil, I want this to be portable, and messy oil and all that will be difficult to carry. If I can come across some money, I might actually look into MMCs .
Plexiglas capacitors are quite difficult for a few reasons. First is that the dielectric must be very thin in order to have a high capacitance, but thick enough to be able to withstand the high voltages. Second you most likely will need to submerge it in oil for temperature and leakage reasons. They tend to be very inefficient and again, very sloppy. If you do opt for this method you can roughly calculate the capacitance by the following:
where A is the area (length * width) of the individual plates, k is the dielectric constant of the material (plexiglas is ~3.4), d is the distance between the plates (i.e. the thickness of your dielectric), and n is the number of plates.
A MMC bank is definitely a great option, but you'll have to ensure you use capacitors that can handle the high voltage and frequency. Many of them break down at high frequencies, so you'll need to find some that can handle hundreds of kilohertz.
If you decide to use a capacitor bank, just keep in mind that capacitors in series have an equivalent voltage of the labeled V * the number of caps, and the capacitance is divided by the number of caps. Capacitors in parallel multiply the capacitance, but the voltage stays the same. For example, 5 1nF, 6KV capacitors in parallel have an equivalent value of 5nF, 6KV, whereas the same capacitors in series would have an equivalent value of 0.2nF, 30KV. Also ensure that your capacitors are rated for at least twice the voltage that your transformer puts out.
Hope this is useful for you. Feel free to ask if you have any more questions
Regards