The most efficient and easiest solution is to just tap into the middle of the 4 batteries. what kind of load are you putting on the 6V portion? maybe if you put enough of a load on the batteries, they will drop enough for a 2.9V output.
Does the laser really need to be 2.9V exactly. I wouldn't see how 0.1V could break it.
OR
you could put a 1 ohm 1/8 Watt resistor in series with it so that if it were running at 100mA, you would get a .1V voltage drop across the resistor, leaving you with 2.9 at the laser. This is assuming that there is exactly 3V going into the resistor/laser and that the laser is drawing exacly 100mA