Well, I have a Polaroid PoGo Printer that prints out photos you can stick on whatever, so I just made a 2x3 print on that.
I took apart the icon from
this tutorial and made it simple. Then I resized it a few times so I wouldn't have to print over and over again. Then I cut it out as a small rectangle and peeled it off and trimmed it a bit afterwards so it would fit better.
You don't want the sticker too thin, otherwise it probably wouldn't come off if you needed to take it in.