Lezyne glueless self sticking patch kit. It's smaller than a credit card and almost as thin. Includes self-sticking patches, a tire boot with instructions printed on it, and a tube scuffer. I've used these for a few years and they worked great. They'll fit into a niche in even the smallest saddle bag.
These used to be readily available for around $3 locally, but may not be available now. I see them advertised on Amazon for $10, which is way too much.
I still carry the same little Lezyne plastic envelopes, but now I'll insert whatever glueless self sticking patches I can find from the LBS or online. Park Tools and others make them.