The only problem with just using a spare button is that if you leave it in the hole it may fall out and if you put it in your pocket you have a problem if you stall the bike or it cuts out and you have to fumble about finding the button amongst the sweet wrappers condoms and old chewing gum ;-)
Attach a lanyard to it and wear it around your wrist..
club card, bottles of oil fuel and water (drinking and coolant).







