First, rinse the rice to remove excess starch, which keeps the rice from getting gummy when cooking, and instead stickier and easier to form into rice balls. Add the rice into a pot or large bowl. Add enough water to cover, then swish the rice around in the water; the water should be very cloudy. Drain into a fine-mesh strainer or colander and repeat this process two more times until the water is almost completely clear.
Combine the rinsed rice and 2 cups of water in a medium saucepan. Bring it to a boil and then reduce the heat to low; cover and cook for 20 minutes.
Meanwhile, in a medium bowl, mix together the turkey and gravy. In a separate small bowl, mix together salt, sugar, and rice vinegar.
Once the rice is done cooking, fluff with a fork and transfer to a large bowl to cool slightly. Drizzle in the vinegar mixture and fold in with a spatula.
Fill a small bowl with water. Lay out an 8-inch-wide piece of plastic wrap on your counter. Spray a ½-cup measuring cup with cooking spray and use it to measure a packed ½ cup of seasoned rice. Plop the rice down in the center of the plastic wrap, then make an indentation with your thumb in the middle of the rice mound. Measure 1 tablespoon of turkey and gravy filling and fill the indentation with it. Wet your fingers (to prevent the rice from sticking to them) and press approximately 2 tablespoons more of rice on top of the filling like a blanket to cover the filling.
Wrap the plastic wrap around the rice bundle and gather it on top. Pick up the rice bundle and start compressing the rice with your hands tightly, like you’re packing a snowball. Press it together, rotating it in both hands, for about 20 seconds.
Unwrap the rice ball and check to make sure no filling is poking through; if so, patch it by wetting your fingers and pinching a little rice and compressing it again. But honestly if a little filling escapes, it’s okay—they don’t have to be perfect.
Wet both of your hands again and shake off excess. Press the rice ball down into a flat disc between your palms (approximately 3" in diameter and 1" in height) and gently press and turn it in your hands, smoothing out any loose rice and cracks with your fingers and re-wetting your hands if necessary. Patch any cracks with more rice, if needed. If you want to make a triangle, make a disc and then flatten the sides and bottom to press into a triangle.
Spray a plate with cooking spray and set the finished rice balls on it; cover with a clean damp paper towel or kitchen towel to keep them from drying out.
After all of your rice balls are made, pre-heat a dry 10-inch nonstick skillet over medium heat for 3 minutes. Thinly coat the bottom of the skillet with 2 tablespoons of canola oil. Add three rice balls to the pan, moving them around so the bottoms are coated in oil. Cook until light golden brown, 5 minutes, then flip and cook for 5 more minutes. (For triangles, cook for 5 minutes and then 1 minute per each side and bottom, 8 minutes total.) Drain on a paper towel–lined plate while you cook the second batch.
Lower the heat to medium-low and add all six rice balls back to the pan. Working quickly, brush the top of each rice ball with soy sauce, flip, and repeat on the other side. (For triangle shapes, also brush both sides and the bottom, too.) Cook 1 minute per side, remove from the pan, and let cool for 5 minutes before eating.
To store, cool completely, then wrap each rice ball tightly in plastic wrap and keep in the refrigerator for up to 3 days, reheating in the microwave for 1 minute to enjoy leftovers.
