I had the same reason to build an UW-torch as each other diver. To make it short the light was unfortunately not cheap.
At first I had to decide which material to use. I chose aluminium
because most nice, commercial, tekkie lights use it. That was only
possible, because I can do turning and have a lathe I can use. I also
made my thoughts about the electronics which should support all
possible features. Of course this thing should look good.
My torch ist made from anodized aluminium. It uses a 12V/3,4Ah lead acid battery and a 35W bulb. More power is possible. It has a 3step dimmer and burns about 65min at full power. The dimmer steps can be set individually. The charger was very cheap. The current can be adjusted. At 1.3A it takes 4h for a full charge. I will integrate a small voltmeter in the remaining space.
It is possible to use a different battery. A 12V/5Ah NiCd battery
will fit. It would have had benefits but was too expensive for the
The switch is a magnetic type from DEV-Pein. The light also has a deep discharge protection which will blink if the battery runs low. There is also a (switch-on) current limiter and a leakage warner. The electronic is from Reinhard's homepage. All features were combinded (dimmer, deep discharge, leakage detector, current limiter). I use a mdeium reflector with 30° - but now I think a 10° might have been better. Never mind the reflector can be changed. It is locked with a plastic ring inside the tube.
The battery is fixed with two plastic rings. One fits pressde
inside the cover. The other one centers the battery. The rings are
glued to the battery with hot-melt-adhesive. The bulb socket is pressed
into an aluminium part screwed to the battery and ring fitting exactly
inside the tube. This way the bulb is centered and protected by the
four bolts. The torch must be opened for charging. A watertight
charging plug (DEV-Pein) could be added later.
total cost for the aluminium €25.(+ a beer for the chief).
or at a shop €?
turning of the thread €35
Parts from DEV-Pein:
Electronic parts €14,50.
Total: €220 could be done cheaper I think. For tests and defect parts there may be another €22-36 and a lot of work.
The torch is bigger than a commercial one, but that leaves space for spare parts in the inside (spare bulb, O-Ring, Silicone paste...)
Good luck for your projects !