I did this years ago in a prototype shop. We used permanent waterproof black ink or (I think) sometimes enamel on light colored rack panels. I'm presently refurbishing a switch box and wanted to try it with white lettering on black. After testing a handful of inks and acrylics, the winner for being waterproof and having reasonable durability was Daler-Rowney FW white ink from Michaels crafts. The trick is holding the ruler and panel- I use some plastic woodworkers spring clamps. If the guide is off the panel I butt another panel up next to it to give an uninterrupted surface. When all the artwork is complete I'll give it a satin overcoat, though I don't know which one yet. It has to be something that doesn't bleed the ink and doesn't look like what it is- a plastic overcoat. eBay seems to have a large number of Leroy guides and tools these days as it's sort of a forgotten art.