Hello James
We always used a pencil never a pen. Go to an art store and purchase a pencil # 8008. This pencil sharpened to a fine point will do the job. It writes on glass, plastic and wipes off with a damp paper towel if a mistake is made. Keep the point fine. we always thought the pencil was a softer line which was more realiistic than the ink.
I cannot send any photos as soon as son returns desk top I will send you photos, if you want.
"Big Art"