D-Size Battery adapter for AA

thingiverse

You can easily convert a D-size battery casing to fit AA batteries by taking into account only their radial size, but you'll need to address the extra length since D-batteries are 11 mm longer than AA batteries. One simple solution is to add more length by filling the gap, and there are clever tricks to do so. I've personally used a ball made from aluminum foil or a spring fashioned from a paper clip with great success.