Those OGM lights are extremely, extremely well made. They use about 0.5 amp when on, which is very little power. I installed an OGM nav/anchor/strobe with photo diode on my masthead. These things are sturdy, small and water tight.
I suggest you not roll your own light or photo switch. Do some searching here for OGM and you'll find many threads where this is discussed. Basically, you want a fixture that is USCG approved. That means you have to use it exactly as it was approved. If you change the bulb, it has to be the same bulb.
This is in case you get into an accident. If your stuff is modified or not approved then you give the person suing you (or who you are suing) an excuse to claim the accident was at least in part your fault.