if you are up to making radio capacitors all you need is foil, it should be spread over "2 opposite sides" of the dielectric material, which is your 2nd ingredient, which can be any non conductive material, thats it
... I bet your capacitors did work, but with such small things charged to only 12v, you wont get much energy out
however if you get your hands on a capacitance meter your device will prolly read a few pico-farads at least
and they would probabally work in a crystal radio kit tuned with a proper coil