They usually have a resistor already, little SMD on the few I've taken a part. Since three button cells are 4.5 volts, you could probably use 4 rechargeables, a diode in series could drop 0.6 volts, if you are worried about the little extra.
There are a bunch of cheap, if not free (if you don't mind advertising printed on the side), LED flashlights, which run off 3 AAA batteries. Mine has an aluminum housing, with plenty of room in the head for the laser portion, but even if it protruded a little, wouldn't matter. It came to mind, because the LEDs are cheap and flicker, but the rest of the flashlight is pretty nice, and kind of fits your needs, battery holder, button, enclosure. Just remove the LED circuit board, solder the two wires to the laser board, bypassing the switch, little hot glue...