There are many pictograms online

We highly recommend that you don't use magnets in this size for glass boards because they easily break the glass plate on your board

Kraftige talmagneter med neodymmagnet bagpå

you could also use an M4 locknut to screw and secure

Uni PWE-5M

Power magnet, Blok 15x15x3 mm. hyldeforkant There are many pictograms onlineNeodymium magnets NdFeB magnets Length: 1. 5 cm Width: 1. 5 cm Height: 0. 3 cm Strength: approx. 3. 4 kg. Flat, square power magnet with incredible bearing capacity. This size is suitable for magnetic glass boards. Please note that flat power magnets may be difficult to move around on glass boards. SPECIFICATIONS: Strength: approx. 3,400 gr. Material: NdFeB Magnetisation: N45 Max. temperaturee: 80C Surface: nickel plated (Ni Cu Ni)