If you have a 300 ma base lamp, you want to use two 1.5 volt D cells in series and a resistor.
The resistor will take .53 volts @ 300ma so you would need approximately a 1.5 ohm resistor at 1/4 watt but I would use a 1/2 watt to be safe.
Now the typical ampacity of a D cell is 12000 ma h and discharge is at 200 ma so the best configuration would be 4 D cells...............2 pair of series cells in parallel. This would give you 80 hours. You can most likely run this light without a resistor but with it the light will last longer.