When I built my Roar of the Pelican, a Maglight 2D flashlight modified to be really bright, I used two cheap 3AA to 1D adapters. I needed 6 AA NiMH batteries to make 7.2 V to power the bi-pin incandescent bulb.
I’d read warnings about some cheap battery adapters not being able to handle the high current, so I bought the beautifully engineered Fivemega 6AA to 2D adapter.
The negative end of the adapter fits inside the spring in the Maglite tail cap. The adapter also has a charging jack that takes a 5 mm coaxial DC power plug, so the pack doesn’t have to be disassembled to charge the batteries. Besides 6AA to 2D adapter, Fivemega makes a 9AA to 3D adapter and 3AA to 1D adapter, among other flashaholic things.
Where did you get the battery adapter