The first few versions of my small spark gap Tesla coil used commercially made capacitors but I wanted to show how to make homemade ones instead. I first made some using soda bottles but they took up a lot of storage space, so I then made some flat plate capacitors instead. Both of these are detailed below, along with videos showing step-by-step how to make them.
This homemade flat plate capacitor is what looks like a small, horizontal board at the base of the Tesla coil below. It has 4nF of capacitance.
For the dielectric I had some transparencies which you can get from photocopying stores, like the UPS Store, basically places that do photocopying and printing for you. Kitchen aluminum foil served as the plates. And looking around I found some thin wood that would act as nice solid pieces to hold everything tightly together as a sandwich. For bolting it together I had some 1/4" nylon nuts and bolts from Home Depot.
My small spark gap Tesla coil was designed for a 4nF capacitor and it turned out that just two plates with a single transparency between them measured 3.86nF, which was close enough. I could do tuning on the primary circuit's spiral coil to make up for the difference.
In the first photo below only the top piece of wood is missing. The two aluminum foil plates have a single transparency between them. The positive plate is the bottom one and has the red wire connected to it, and the negative plate is the top on and has the black wire connected to it.
Below is a video showing step-by-step the construction of this capacitor, along with some variations and demonstrations of it in use.
I decided to try the large 2 litre/2 quart soda bottles since the large surface area would give a reasonable capacitance and the plastic has a high breakdown voltage. Also, once you cut away the top, they have a fairly open end for reaching your hand into for putting an aluminum foil electrode.
Each soda bottle capacitor ended up having a capacitance of between 1nF and 1.4nF, depending on how close the aluminum foil was to the plastic everywhere.
The following is a video showing step-by-step how I made these capacitors, along with demonstrations of them in action.