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Which is basically another way of saying it's hard to believe what a difference a properly shaped light makes.
very true! At this time, there isn't much advantage to making your own headlight, other than desiring special features. Even then, it's probably best to find a damaged commercially built light and modify the electronics (and I've got such a light ready for a project, eventually)
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... I eventually settled on bright LED's on the little hexagonal aluminum stars; you can get excellent optics for those. The LED plus the 'lens' (it looks nothing like a lens. There's another name for it that I'm not going to bother googling right now) forms a cylinder about 3/4" long and 3/4" across, which is pretty easy to fit into any old dynamo lamp.
At first I fooled around with bridge rectifiers and voltage limiters of various kinds, some of which I probably didn't understand anyway (I'm an archaeologist, not an electrician). .....
As you note, it's not that hard to put together a reasonably decent light with little specialized knowledge. I've made a few over the decades and am still using two of them.
I'm an electrical engineer, so it's natural to fool around with the electronics. I seem to have focused on very strong standlights, though.
Retrofitting "subtle" LEDs into retro/vintage lights is an interesting variation on the theme, though. The problem is primarily mechanical, at least in terms of getting the heat out of the LEDs and getting them positioned properly in the existing reflector. I don't have much for tools, so this will involve a lot of work with the hacksaw, file, and probably the Dremel. It's an intriguing problem, regardless.
Steve in Peoria