It may take a bit of tweeking with spacers, and making sure everthing contacts, But once you get it, your good to go. You can swap them out over a...
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.