Check out Fenix flashlights, I have the TK-12. I put a butler Creek scope cover on it to protect the lens and keep the recess free of crap.
The mounting system I chose is made by Tac-Star, available for just over $30 and uses a stem to mount the light in the sidesaddle. The flashlight can be either 7/8ths or 1" and is held in place by a scope ring on the top of the stem and a large locking screw on the bottom keeps it secured. It does not interfere with the shells in the sidesaddle provided they are inserted from the bottom. The flat side of the scope ring lies against the side of the sidesaddle preventing it from turning, thus when on the light beam illuminates the front sight and the target. However it does need to be checked for tightness from time to time. It is quick and easy to mount or dismount and the position of the light is far enough to the rear of the gun that you can operate the light with your support hand rather than with the pressure switch; in fact the wire from the pressure switch is a bit of a nuisance on the 590 with this particular flashlight mount. Google Tac-Star and choose flashlight accessories from the list.