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).

Step 1:

Start by gently prying open the top of the battery case, being carefull not to damage or tear the metal.

Step 2:

Carefully pry up the cover with the springs, and you will find 4 "oversized D Batteries" in series to add up to 6 volts. There is a piece of cardboard with metal strips at the bottom to bridge the positive of one battery to the negative of the other, and a strip on the top to connect the positive of one bridged set to the negative of the other. There is also a metal spacer that is pictured in step 3.
As you can see, a D battery is shorter (by 1 1/8 inch).

Step 3:

I used the bottom of a water bottle with some cardboard on top of it as a spacer. I then reinstalled the metal spacer and the cardboard with the strips of metal.
Because the negative side of the d batteries in indented, it did not make contact with the metal strips so I had to make spacers out of the metal button snaps I had laying around. I used dielectric grease to hold them in place. Other things can work if you don't have snaps. (If you got this far, I have faith in you)
Next you install the batteries alternating Pos, Neg, Pos, Neg, so that you end up with 2 sets of batteries bridged at the bottom.

Step 4:

Now, install the top cap. It is very important that the bridge bar on the cap is perpendicular to the strips at the bottom, so that you connect the 2 "bridged sets" together, Positive to negative. Test with a meter shows just over 6 volts.

Step 5:

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 and over again.