I have a 35w and I use 20s/100p for cermark. If the mark won't stick even after all you have tried, maybe you should talk the customer into an identification tag.
I got these from the lunatickstore dot com or from amazon dot com.
I have bought several of these. I have had to take the piece they include and copy the shape and use my own material (silver flexibrass). This might just have to do.