I was still looking at the remaining ping pong balls, and wondering where to put them away, when I spotted a string of fairy lights looking unloved. On went the figurative lightbulb, and the Lucky Charms appeared!
You will need:
A string of fairy lights (and batteries if required)
A pair of scissors
A handful of ping pong balls (I only had enough for every other light, it doesn't matter if they're all covered or not)
Step 1: Piercing
You need to pierce each of them, and then file/cut out the hole so that it is slightly smaller than the diameter of the light bulb. I used the blade of a pair of scissors, and turned it around in the hole, to carve out the excess plastic.
Be careful with this - I slipped on the one in the top right, and made a huge slot, luckily, it wasn't so big it doesn't fit now!