I thought about doing this but then you're committed to Li-ion. It's not too much to expect an LED flashlight in 2022 to take both.
Malkoff isn't using dated drivers in 2022.... Malkoff wants absolute reliability.
The only custom maker I know offering a driver that both boosts and limits the voltage is HDS.
Most custom makers require a 3.7 Li-Ion. If you've held an HDS you'll notice it's a really long light for its battery size. That is because to truly do a reliable duel-fuel light you need the room for all the gadgetry.
The chinese imports claim to have "duel-fuel" capabilities, but they won't last a lifetime like Malkoff and they won't hold up as a WML.
99.99% of the chinese imports aren't really duel-fuel like they claim....that's why they get super hot when you use a Li-Ion. They still send 4.7v to the 3v LED and eventually it fries then you buy another one.
But me and
@bykfixer gave you the solution you were after.... a 3v rechargeable 16340.