In our town there is a shop selling outdoor clothing and army surplus boots, bags, jackets, etc.

I bought a dark green (army style) canvas bag about 10" x 5" and about 6" high for about 5. I cut a piece of foam to fit inside at the bottom (about 3" high) and cut out recesses to take a 35mm SLR fitted with its 50mm lens and also spaces for the 28mm and 135mm lenses and a couple of spare film canisters.

This looks nothing like a camera bag and is small enough to carry without it getting in the way.