You can actually buy ringlights made in that manner,
and they work
pretty well. There are usually some cheap ones on a famous "auction"
It is my experience that just about anything intended for photograp[hic
use is way overpriced :-=). OK, I can't grind lenses, so I am happy to
pay for good ones, but I am not going to pay high prices for things I can
make in an afternoon or so. Quite apart from the fact that making the
device and experiemnting with it is more fun to me than taking photographs...
-tony