Piston trimmers are great.
Most have one terminal to the mounting bushing (normally grounded) but some have two leads for non-grounded operation.
More expensive ones have sapphire dielectric between the internal moving piston and the outside cylindrical electrode; cheaper ones use glass. Others are PTFE or air dielectric.
If this be a onesie (not manufacturing) project, Surplus Sales of Nebraska has a good selection of pistons.
https://www.surplussales.com/Variables/PistonTrimmers/index.htmlFor example, on the "high-Q" page, their catalog no. CTP5452 is a Johanson 1.0 to 16 pF panel-mount unit, air dielectric, with
Q > 3000 at 100 MHz, for $18.99.
On the "standard" page is CTP MPT62004A from JFD, Q not specified, 0.8 to 18 pF for $6.00
Both of these have enough change in C to reach your 25 to 40 pF range with a suitable fixed capacitor. Higher capacitance units are available there.