I made about 20 at home out of an old tire one day and left them sitting in a drawer in my garage/shop.

I always carry one with me, simply because we've actually used them on group rides out in the mountains 75km from the start of the ride... Helped get a young lady home once - she was very grateful and made me some snacks as a thank-you.

I've got a nice-size seatbag just for things like that, and it's not like it weighs me down...