Check out Carey's Creations.
3"x8" Vinyl Decal of your call sign.
These are a 6"x6" magnet.
Most of the events I volunteer with are bike rides, so its nice to let the other drivers on the road know what is going on ahead of them.
12"x12" Magnet Bike Sign
Use this 8"x12" Magnet sign to let everyone at the event know that you are part of the Communications Team. These can also be made to say "Event Communications".