It is always a good idea to have one and it is very satisfying to make one! All you need is a knife that is about to end in a bin. And some tools.
Cut and prepared for polishing. The knife doesn’t have to be insanely sharp. It’s just a marking knife, single bevel because I’m right-handed.
The drill rotates slower than the angle grinder and this is crucial to get a fine polish. Grits from 40 to 3000 were used.
It works. A quite nice line is marked and ready for cutting or chiselling.