One of the best homebrew tips passed on to me (by Dave g3zxx) was using open ended lengths of coax stub as a capacitor in single band impedance matching (parallel tapped coil and cap). It's the nature of coax to take high voltage without arcing and therefore several KW of RF power, so it out performs a air spaced tuning capacitor in both performance and cost. Once tuned the 'variable' isn't needed it's just handy in the initial set up so would be a waste of an expensive capacitor. Multi banding could even be achieved by attaching different pre tuned length of coax stub maybe via quick change BNC connector.

I am also interested in making a variable capacitor from coax, or at least one with a little tuning range to help in the initial set up so am researching that for future projects. Thinking along the lines of coax inside a sliding copper pipe sleeve. As it stands once the stub is trimmed near the required resonant frequency a little fine tuning can be had by removing about an inch of the insulator sleeve at the end of stub and bunching the braid backwards and forwards to find the sweet spot - then taping up to secure.

This post is something for me to refer to edit and add to in future...

"RG-213 equals about 50 Pf per foot of capacitace, which means every inch of coax is worth about 100-200 KHZ"

I measured Approx 15 foot of rg316 coax = approx 600pF which sort fits in with the above statement.

Longer the stub = greater capacitance = lower the frequency, trim stub to raise the resonant frequency.