Your problems with shielding your eyes from light probably come because it is easy to shield them from direct light from the car lights, but hard to shield them from reflected car light coming from every angle.
- I suggest that you experiment with closeing your eyelids while shining lights at your face.
I just tried shutting my eyes while looking at a light about a foot and a half from my face. Everything looked a very dark grey or black color, and it didn't look any brighter when I looked at the light bulb.
I often close my eyes when a passenger in a car and facing toward bright sunlight. The sunlight shines red through my eyelids and I tend to see geometric patterns - when the car passes into shade from trees the color changes to blue. So the geometric patterns sometimes flash between a red and a blue background rapidly.
But looking at the light bulb right now I don't see any light shining through my eyelids, because sunlight is many times as intense as the light bulb.
So possibly you might want to experiment with staring at lights of varying intensity with you eyes closed to see how much light, if any, penetrates your eyelids. And yu can try calculating how bright a car's lightswill be at your distance from the road, and comparing it to light sources which doo or do not penetrate your eyelids.
Maybe simply closing your eyes is enough to prevent loosing your night vision.
You could wear a helmet with a hinged opaque face plate that you can slam down when you hear a car coming. You could have opague cloth handing down several feet from the bottom of the helmet to prevent light from getting in through the bottom, or maybe tape the bottom of the helmet to your clothing.
If you own the place you observe from, you could built a tiny windowless shed or shack there and make certain it is pitch black inside with the door closed, and run into it and close the door when you hear a car coming.
You could set up a tent made of totally opague fabric or plastic sheets, and when you hear a car coming jump into it and knock down the tent pole to collapse it on you. When the car passes, struggle to get out of the tent, perhaps by finding the tentpole and putting back up.
I hope you might find some of my suggestions useful.
