Nisonger makes, or made them. As far as doing your own, I started to do some, as I had done them on other cars, and although I THINK it could be done at home, there's a LOT of pitfalls, the biggest of which is that your guages aren't flat. There's a top and bottom piece. The top has the cutout, which would have to be accounted for, and the bottom is shaped like a well, which would also have to be accounted for. They're also VERY easy to break when you dismantle them, and you have to dismantle them to do the job.
When I've done them in the past, I've done an overlay kind of thing, but I don't know if it'll work on Smiths guages. It's still on my to-do list, but I've been kinda busy job hunting lately and haven't been able to fool with it.