I'll second that 3/32" allen wrench. And as Bob pointed out, DON"T OVERTIGHTEN the locking screw. Garmin made them with a brass screw and once you strip it (easily done), it's duct tape to hold the radio in. Or disassemble the whole radio case. That's why the SL40 comes with a screwdriver style allen tool; supposed to make overtightening more difficult. However, where there's a will there's a way.
George