This instructable will show you how you can still use your 6 volt lantern when you only have D cell batteries. A single D cell battery is 1.5 volts so 4 of them is 6 volts. You will need an old 6 volt lantern battery, 4 D batteries, and some materials to make a spacers. (I used a water bottle, some cardboard, and some metal clothing snaps).
Start by gently prying open the top of the battery case, being carefull not to damage or tear the metal.