I have followed the thread about variable capacitors recently and would
like to suggest a piece of simple but effective software that will
provide info about building your own home brew capacitors using a
variety of spacings and dielectric material to get the capacitance you
might need.
You may find this on the website of VE3SQB at www.ve3sqb.com
Scroll down the screen until you see the title "For the Experimentor."
The mechanics of building are up to you, but I have found it to be a
great help in designing.
de WA2AFD