For optimum performance (High Q) the loop should have a low electrical resistance and the tuning capacitor low losses.

Please be careful: Even with just a few Watts transmit power the tuning capacitor must be able to handle several hundred Volts and currents of a few Amperes.

The cheap option:
Air gap tuning capacitor with both sections used in series to avoid the use of lossy sliding contacts.

Vacuum Variable Capacitor tuning circuit:

I got this cap for a tenner at a junk sale - a real bargain. Brass plates and SO239 sockets connect it to the loop conductor. The big tuning knob came from a dead rice cooker ;-)

I removed the centre connector of the SO239 and PL259 connectors by gently heating them up and then punching the plastic out. This avoids smelly surprises when soldering the thick pipes to the connectors and the sockets to the brass plates. The centre connectors are not needed anyway because most of the current is flowing through the surface of the loop (skin effect).