I use both a old diaper bag and an REI lunch bag like the one in the above link. The lunch bag is large enough to fit my Bronica ETRS w/speedgrip and an extra film back. Nicely cushioned also so everything is protected.
I also use an old diaper bag backpack as it is covered by padding for keeping things cool and is waterproof. I like that the diaper change padding to lay a baby on a table is stored on the side facing your back for extra comfort. The pocket inside the bag for a milk bottle now can be used for a lens or a flash. Best part was that it was free since my kids are bigger now.