I have seen some designs of homebrew variable capacitor, one of my favourites is the piston type using copper tubing and coaxial cable. Why? Well it looks from the mechanical point of view the simplest to make and the most forgiving to mechanical tolerances.
Said that, here's some tests I made thinking in building one of 300pf for a long waiting antenna tuner project.
Movable part will be the coaxial cable... well correctly speaking it depends on your reference point of view...
I only have to found a smaller diameter cooper tube since this one used for tests leaves a 1mm gap to the coaxial cable and I want a tighter tolerance.
Only have to and build 10 similar pieces to get the expected capacitance.