I once made capacitors using flourescent tubing that comes in a variety
of shapes including trobone. I coated them with epoxy plus added
aluminum wrapping foil to the outside.
Used a series of straight forms once for a three element yagi where
the capacitors resided inside the boom and was driven by a single
stepping motor to get linear movement. for resonace/frequency change.
The arrangement worked quite well despite the fact I used a aluminum
boom !.
Art