A vactrol is a light depending resistor (LDR) placed next to a light source (LED). As the light gets brighter, the resistance goes lower. This is a great way to add voltage control.
These two components can be hold together using a heat shrink tube. Since these LDR's are 5mm, I would suggest to use a 6mm heat shrink tube, that shrinks down to 3mm (the LED).
