Glue the face of the photocell to the side of the neon lamp.  We prefer to use a drop of thick gel super glue (cyanoacrylate) for holding the photocell to the side of the lamp. It works great and is clear so it doesn't obstruct the light from the lamp to the photocell.    Allow the glue to dry for several hours, or if using a super glue type of adhesive, you can spray a little super glue accelerant to speed up the drying process.

Finally, carefully bend the leads of the photocell at a right angle so they are inline with the neon lamp leads.

Now to encapsulate it.
 

Slide the tubing on the lamp/photocell combo, centering it between the leads sticking out.   The end of the tubing with the silver dots must go on the side with the neon lamp leads - this is very important.   When installing the device in your Fender amp, the side with the silver dot (neon lamp side) installs closest to the "tube" side of the chassis, while the photocell side goes toward the potentiometer side of the fiberboard.   If you should reverse the opto-coupler when installing it, you will ruin it when voltage is applied, so be certain before you encapsulate it.    Mark which side is the lamp side in some manner.    Before starting to encapsulate, make sure that the leads are separated, and not shorted.

The heat shrink tubing will shrink to about half its diameter when properly heated. Use a heat gun or a hair dryer and apply even heat all around the tubing, holding the heat nozzle 6 to 8" from the tubing.

Use A NON-WATER-BASED opaque adhesive to fill the ends of the tubing. The adhesive should have a high electrical resistance (this is the reason for using non-water-base type), since the OFF resistance of the photocell is over 20 Meg and we don't want leakage through the adhesive to cause crosstalk in your circuit.  It is very important to block the ends of the tubing with opaque glue!   Preferably, use a thick adhesive so you don't have to deal with it running while it is setting up.   A black silicone sealant (non-water based) makes a good choice.

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