I love everything about this cardboard guide that came with the ergo pouches we ordered! The heat sensitive strip highlights the current temperature in green and requires no batteries. That temp corresponds to the chart below which is color coded to make sure you don't dress your baby too warm or cold.
It's thin, light, cheap, easy, and even has a hole knockout at the top for wall mounting and a cutouts at the bottom for free standing.
Such a great design!! Whoever created this, you have a fan for life!