I use the Tenba BYOB 10 for my Timbuk2 backpack. It perfectly sized to hold a camera, one big zoom lens, a prime lens, and a few miscellaneous items. I fit an X-Pro2, 16-55/2.8, 23/2, and X100T in mine. It’s reasonably padded, easy to access, and a great option to swap out of existing bags.
If you want absolute smallest, the BYOB 7 or 9 would probably be a good fit.