I would be temped to make a hole larger, use styrene filler, and plant the fitting in the filler before it set, then hide it under the paint. Sometimes I really am glad that I'm not a craftsman...
There used to be a slotting tool that looked like an engraving tool fitted with what looked like a short piece of junior hacksaw blade with the teeth at the end. Downside is the end was probably bigger than the holes needed. Drilling looks the best option, but I would do my initial holes with an Archimedes pin drill, and use the inevitable broken bits to form the slot in an electric drill run at low speed with water for cooling.