I've been wanting to make labels for my sanwu lasers using @Trevor 's excellent laser label generator. It's quick & easy, costs almost nothing, and should be strong enough to last for a while.
First, print the label. I printed it just shy of 2 inches long, large enough to be noticeable.
Fold it exactly in half, then trim the excess white if you want it to look neat.
Loop a piece of string and tie a knot at the end. Tape it to the inside of the label, them trim the excess tape. maybe put a tiny bit of superglue for extra strength?
Fold the label, then tape the whole thing around the opening of the label. Trim the excess tape.