I usually just secure these little motors to plywood firewalls with small wood screws, like the ones that come with 9 gram servos. I've never had one come loose.
Remember that it's often easier to drill the motor mount out a little bigger to fit the screws you have than to keep a stock of odd screws sized for every motor mount. Some are metric, some imperial depending on brand. I usually use 6-32 and 8-32 to secure larger motors, and drill the mounts to suit.