Something like this https://www.amazon.com/gp/product/B004WLCLQY does work to an extent, spreading the beam out along one axis.


If you'd rather do your own, and it actually does work better, start with a full page fresnel lens like this https://www.amazon.com/Premium-2-Pcs.../dp/B01HP48ZAK (not a specific recommendation, just an example that I picked randomly). The curious thing about those magnifiers is that they change the beam differently depending on how far you are from the center, which is why I start with a full-page size. Near the top it will spread out sideways more than vertically, and also angle the beam. So just hold it against the light, move the light around until you get the best beam shape you can, then cut it out with scissors and put inside of the headlight lens. You can start with a smaller magnifier and get more spread, but less ability to aim and shape.