mrfixitman
Enlightened
I have used 3 or 4 A123 3.2vdc batteries in 3 or 4 D Mag-Lite I upgraded bulb to match higher voltage. Just flip the end ap spring.
There has to be PVC the right size to adapt c to d cells.Adapters are not commercially available but some people can easily make them even in their garage with lathe.
Voltage of single Li-Ion is pretty close to 3 cell "D" (Alkaline or NiMH) but for 2D size, you need to make sure your module will take voltage of your new set up.