3C:
You should be able to stuff in six 1/2 C size 850mah NiMh Cells
from batterystation, change the reflector to a aluminium SMO
or LOP and run a ROP HI bulb for that really powerful tight beam.
OR
Use a spacer and 2 LG 18650 Li-ion batterys to make a ROP HI LE
(lithium edition) with the same reflector and bulb as above.
2D:
6AA-to-2D holder, CBP1650's, same reflector and bulb as above
Then there are lots of more bulbs to choose from if you use KIU
socket instead of the orginal socket, but then it does not meet
your condition to not mod in any way, The reflector is a must
to change with bulbs more powerful than ~10w or it melts.
If you want you can run the ROP LOW bulb for (about) half the
brightness and double the runtime, but still lots of light.