MY DAUGHTER UMPIRES HOCKEY AND NEEDED SOME RED/YELLOW CARDS, 

we re-mixed these with a 6mm hole for attachment to carabina, lanyard, cord or rope.

these are durable and reusable, print in red, yellow or other color filament as required ... petg will last longer, ASA and ABS are even better but pla is easy and works fine...  2 walls is enough and no infill is used as its only 0.8mm thick (making it light weight and flexible)