I agree, it's a minefield of information. I'd love to write a guide but I just don't have the time, and.....
Unless you have access to a lathe or vertical mill making the housing takes a lot of ingenuity or becomes prohibitively expensive. Modern lights are so good and so (relatively) cheap that it's pointless making your own any more. That's before we start getting onto the safety of making your own battery pack and charging it safely.
Without trying to sound like a condescending arse, if you know you know, if you don't, research the relative forums or please buy something off the shelf. Dentists are expensive, I've had one of my lights fail once, for a stupid reason and it was a horrendous experience on a fast Welsh descent. When my home made one dies I'll probably buy something from Gloworm in New Zealand.
I've fixed many sets of friends generic Chinese lights over the past decade. You couldn't pay me to ride with one unless I had backup. Or charge the thing anywhere near my house. Magicshine are the only decent ones. I struggle to understand how some people are willing to spend £900 on a rear shock (for instance) that is only marginally better than their old one but recoil in horror at the thought of spending £300 on a banging set of lights, and are happy trusting their night riding to a generic 7543000 lumen xxml t18 Chinese light.
This is the best of the cheap generic Chinese lights I've seen. It comes as just a light head, you need to pick a battery and charger from the same website. Still wouldn't charge it in my house though.
KDLITKER BL70S Cree XHP70_2 3000 Lumens 4-Mode High Power LED Bike Light
kaidomain.com