It's not good.
I've finished dremelling, sanding and polishing, and it looks like this:
Not good at all. I didn't have a depth guide so I did it all by hand, and this has made the surface uneven. I can't seem to sand and polish it to a reflective shine at all. So there we are, it's fit for the bin.
At least it was a spare that I bought for this very reason. If I want the projectors, I'll have to do it to my main headlights. Good thing about that is the indicators are already clear. Bad thing is that they're very expensive, and I don't think I'm brave enough to split them open. So it looks like this headlight project may have to be sacked off. I'm gutted