Made By Monkeys

You will require one large fake slug (don't use a real one), a small fibre optic loom like those found on kiddies' flashing wands, a couple of bits of heatshrink sleeving, a resistor, an LED and a cheap USB phone charging lead.

The lead I used was for charging a Nokia, but was employed purely for its availability. If you use a standard USB lead, then you can get power from the thicker wires, but must ensure that the two data cores are insulated to protect from damage.
The resistor value is based on the USB supply being five volts minus the forward voltage of the LED, which in this case is about three volts (blue). This gives us two volts to drop across the resistor at 20mA, so 2/0.02 equals one hundred ohms.

The fake slug is carved out with a Dremmel to accomodate the fibre optic loom and the electronics. A small hole is drilled in the end to allow the cable to enter, and the slug is peppered with small holes to accomodate the fibres.

A piece of heatshrink sleeve is shrunk onto a five millimetre drill shank to gain a friction fit for the LED. A drop of two-part resin is applied into the end of the sleeve, and the fibres are carefully inserted fully one at a time. When the resin has cured, the USB lead is terminated to the LED via the resistor, and sleeved appropriately.

The fibres are individually threaded through the holes in the slug, then the whole lot is carefully pulled into position in the recess. The harness is secured internally with more two-part resin, and when it has cured the fibres are cropped to within 5mm of the slug's surface. For a neat appearance, the ends of the fibres can be touched briefly with the side of a hot soldering iron to make them bead into a small lens. This also stops them from popping back through their hole.

And voila -- the finished product! And this is one slug that won't eat your marigolds!