Just FYI - I found some hobby grade plywood, 6mm thick - about the same thickness of the rotors on my bike. I cut out a section shaped like a brake pad. If I have to remove a wheel, I just slip the wood between the pads, so if I accidentally press the brake lever (easy enough to do when changing a flat in the rain out on the road. Trust me) it's no harm, no foul. Just remove the wood and slide the rotor back in.
The wooden block is small enough to carry in my under-saddle bag, along with my spare tube and tire levers.