At last I've found a use for all those old 'tetrapack' soya milk boxes (same with fruit juice boxes). I'm going to cut the bottom and top off the box and then cut the sides into strips to make decent sized waterproof plant labels. Waterproof markers seem to write onto the insides OK and a small hole (made with an office hole punch) means I can fix them to the ground with a tent peg or knitting needle.