They are escape artist. Call local rescues, vets, pet stores and let them know you found a Russian tortoise Incase anyone calls them looking for one.
Also put an ad in the local neighborhood app and Craigslist.
Make the people who try to claim him prove it's theirs by showing you a pic and describing him.
Good for you for rescuing the tortoise before it was hit. Hope you can find the owner soon.
A plastic tub or a bin like a Christmas tree storage bin would be better than a cardboard box for keeping the tortoise for a short time. Cardboard will get wet and fall apart. Put a 4" tp 6" deep layer of damp cypress mulch on the bottom of the bin.
The tortoise also needs a water dish so it can drink. Use a clay plant saucer so it can get in and out easily without tipping over. It also needs green matter to eat (grasses, weeds, etc.). They will also eat dandelions, clover, and opuntia cactus pads. You can feed the tortoise on a flat piece of slate placed on the substrate or in a second clay saucer.
It is also a good idea to soak the tortoise in 1" or so of lukewarm water in a smaller tub (from which it cannot get out by itself) for about 20-30 minutes daily to keep it well hydrated.