As mentioned in the comment by henryflower, make sure that the shaft (and thus the frame) of the variable capacitor are grounded.
You need to use rubber mounting grommets around the screws which bolt the tuning capacitor to the PCB or chassis. Then run one thick wire from the frame of the tuning capacitor from a point away from the front panel to one point ...
Circuit looks OK.
Note that the highest impedance point in this circuit is near the box open end. RF voltage here is highest in this oscillator, and is susceptible to coupling out the top.
A shield lid (soldered to the box) should help.