I just got this assembled last weekend.
printables, complete with F3D file.
This is a battery adapter that will allow you to insert D cell batteries into a LiitoKala Engineer Lii-500. I included the Fusion 360 file, so you can customize it to fit other applications, but odds are it will fit most 4+ 18650 based chargers that are also compatible with Ni-MH.
It prints in two halves that I simply hot glued together around their perimeter. It's not like this part is going to see very high loads… The parts are designed to print without supports. My copies are 20% infill (you could easily go with less) and 3 walls for an overall wall thickness of 1.16. Wall thickness is something I wouldn't change, at least for the battery side so the battery terminals don't eventual fail. The batteries are a very slight interference fit to ensure a robust electrical connection.
For terminals, I used 8mm wide nickle strip. I'm sure you could use something else, but I had some on hand and it seemed like an obvious application. Connections between the two halves were made via wires and solder.
The chargers typically support 3 sockets, you could adapt one to fit two big D's for Double Power.