Ten Tec in USA offers a variable capacitor kit, max C is 500pf @ ~5kv.
Cost is about $80 USD. Takes about a hour to assemble. I use them in
a 1.5kw ATU and they work fine. Kit #1207.

If you get all the parts, cut them up, and assemble the whole thing,
homebrew, makes the $80 price tag seem reasonable. I could see you
spending more than half on just materials.

Good Luck,
Gary N4AST