A varactor also needs a high enough bias voltage to get the desired capacitance. I thought of suggesting a small 12V battery (the type smaller than an AAA and often found in key fob transmitters) with a high value pot. 1M but then I realised the value would change as the battery runs down which might not be a problem if it's designed to be adjusted frequently.
Is it a trimmer?
You could connect different value capacitors in parallel and a DIP switch to connect ans disconnect them. For example if you permanently connect a 15pF capacitor in parallel with 47pf, 22pF, 10pF, 4.7pF and 2.2pF, each in series with a switch you'll have a 15pF to 100pF variable capacitor with roughly 2.2pF steps, even better you could replace the 2,2pF capacitor with a gimmick capacitor which will give a smoother adjustment. I'll post a schematic if you're interested.