I needed a guitar pick so I slapped this little bit of code together in OpenSCAD and tried to make it at least tolerably customizable. You'll want to adjust settings for your printer's layer height. I was shooting for a 360um pick which is extra, extra thin.
Font size of 4mm seems adequate for printing with a 0.4mm nozzle.
Edit: I have no idea why but this thing isn't working in Customizer but I'm leaving the SCAD up in case anyone feels like installing OpenSCAD and customizing it themselves. I've also generated a handful of STL files so you won't need to customize the picks to get a variety of thicknesses.
Be very careful peeling this off the printer bed. I was astounded it came up without breaking or warping at all.