
I use standard double knit wool.
One strand put around the neck and tied in a knot, leaving the two ends to make a tag.
This is then easy to pick out in photo if you go for bright and contrasting colours.
I leave about two fingers width of growing room, and change them probably weekly.
By the time they could catch on anything once mobile their weight (unless tiny bred) would break the strand.
I used to worry about strangling, but many many people have used this method in my breed and others.