Step 17: Secondary light sources - Assembling light pipes
I did not want the LEDs to poke straight through to the front. For one thing, 5mm is too wide! Also, the substrate was too thick for an LED to reach all the way through. Instead, obtain a few meters of clear acrylic rod. It conducts light like an optical fiber, only it's 2-3mm in diameter.
Put together a pile of as many LEDs as you'll need, a pile of 2cm-lengths of heat-shrink large enough to go over an LED, and a pile of 2.5cm-lengths of acrylic rod. (You can quite easily cut the rod with a good pair of side-cutters)
Assemble the LED, heat-shrink and acrylic rod together, then using a soldering iron or a heat gun shrink them together. Try to avoid getting the LED too hot, as it can kill them. Repeat until you have a hundred or so of these assemblies. (This is, believe it or not, another step that takes a long time.)
Note: I initially tried melting acrylic rod directly onto the LED. The LED is a different type of plastic so the rod does not 'stick' properly, the acrylic starts to darken when it is heated, and lots of noxious gases are produced. Not recommended.