In a large nonstick skillet with a lid, stir together 2 cups water, the rice, 1 tablespoon rice vinegar, the sugar and 1 teaspoon salt. Bring to a boil over high heat, then cover, reduce heat to low and cook until rice is tender, 18 to 20 minutes.
Thinly slice the cucumbers, sprinkle with a pinch of salt and set aside.
In a large bowl, stir together the tuna, mayonnaise, Sriracha, soy sauce and remaining 2 teaspoons rice vinegar. Taste and add more Sriracha for more heat. Refrigerate until the rice is ready (or up to 5 days).
Now crisp the rice. Place oil in non-stick pan and heat until simmering. Add frozen rice and cook until the rice is browned at the edges, 5 to 10 minutes. Lift the rice to check that the bottom is browned. Slide onto a plate, cover with a second plate and flip the rice cake over. Add more oil to the pan if needed and slide the rice cake back to continue browning.
Divide the rice, crispy side up, among bowls or plates and serve with a scoop of the spicy tuna and the cucumbers.