As you can see, a D battery is shorter (by 1 1/8 inch).
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.