Keypeg for Cello
The key is designed to increase leverage by means of surface area, so it is best to place the key in tha palm of the hand and close the fingers around it.
Set yorself up so that you eliminate any possibility of finding yourself without a key, or spare peg. Keep a key in your case, one on your keyring, and even keep the old pegs in your case. If you find yourself owning just one key, consider ordering one more from your luthier.