Two comments: there are a few home-made low-rise rectangular conversions around that use only the low beam (or high beam) bulb of a std 4-headlight system from the '80s. Looks nice but limits your safe speed after dark. There's no law that demands you have high AND low beams, only that if you do, they be aimed properly.
And some modern bulbs use plastic lenses. These are slightly lighter than glass, but turn yellow after a year or so, locking you into constant polishing to restore function. Too much work for me even with the (small) weight savings. Suggest you stick to the vendor systems rather than trying to save a buck by DIY.