LED light source is widely used in these days, chose the right LED version for you is the first step.
Chose right light source after got the Lumen request of the flashlight.
Power System-
Dry battery power system & Rechargeable Li-ion battery system is popular nowadays.
Based on the autonomy time request, You can chose multi-battery power system.
Work mold-
The tradition working mold is High, middle, low, strobe, SOS signal. The details as below:

Focus mold-
LED Bulb focus, Zoom in/out focus by LEN, Concave LEN.
Other Special Requests-
Water proof for diving, Special Lighting cover(such as UV), With Tools……

Feedback to OTC R&D to enjoy DIY service.