I’m not saying don’t build one, I love the creative builds people show here on BLF, but for your intended use, the last thing you want to be doing during an emergency, is messing around trying to crank charge a light. An ‘off the shelf’ light is likely to be more waterproof, and potentially more reliable than a self-built crank light.
CR123 or AA lithium cells used to be the go-to ‘fit and forget’ power source for emergency lights if you’re worried about cell discharge.
Li-ion cells generally hold charge quite well, I’d say that a Convoy S2+ with Sst-20, 3x7135 option and a decent 18650 or two would be suitable.