If a hobby store sells them, I think it would be easier to make the visor parts using aluminum tubes and use an electric drill as the tool. Just cutting that angled part of round styrene would give me a sore neck within 5 minutes, it's hard to make all 3 the same, and they all might not match up with the rest of their gauge bodies the same way.
Find a tube with the size that you can fit right over your plastic gauges. Angle one end using needle file (put file on a table and grind your tube end on it), cut if off by some length (more than what you need so you can put the piece into a drill). Put the piece into a drill. While spinning, thin angled edge from inside with round needle file/ or thin entire tube from outside with flat needle file, sand/polish off file marks with sandpaper, finally, saw off the tube length you want. Afterwards tuck your round plastic gauges into the aluminum tube, then the piece is ready for paint.
Similar to what buggy2356 is saying above but just going deeper into the subject. After trying the power drill lathe method I can't stop using it! It's fun and you can make the final product so thin it's hard to do the same with styrene due to the difference in strength. Exhaust tips made this way look excellent compare to styrene tips as well!