Combine the rice vinegar, sugar, and salt. Mix until dissolved.

While the sushi rice is hot, season the rice with the prepared rice vinegar and furikake. Fluff well so that the seasonings are evenly dispersed. Cover with a damp towel and set aside to let the rice cool.

Meanwhile, place the drained canned tuna into a mixing bowl. Add the Kewpie mayo and Sriracha and mix until evenly combined.

Place half a sheet of nori, shiny side down, and place the tip of the triangle mold at the center of the top edge of the seaweed. Add enough rice into the mold to cover the bottom, about ¼ to ⅓ cup of rice. Spread the seasoned rice as evenly as possible on the bottom and slightly up the sides of the mold to form a little 'bowl.'

Place about one tablespoon of the spicy mayo tuna in the center and cover the top with about the same amount of rice as the bottom, about ¼ to ⅓ cup. Spread out the rice if need be, then use the press to firmly and evenly press down the onigiri. Keep the pressure on the press and pull the triangle mold up to release the mold.

Fold the bottom half of the nori over the onigiri and fold the corners down to adhere. Set aside and repeat with remaining ingredients.

Place a sheet of cling film flat on your work surface and spread about ½ cup to ⅔ cup of the seasoned rice at the center of the cling film. Try to keep to be as even as possible and keep the rice about ½ inch thick, but make sure there are no holes.

Add about 1 tablespoon of the spicy mayo tuna at the center of the rice. Next, using the cling film, bring the rice together and twist the cling wrap to form a ball. Undo the twist and gently flatten the rice ball to about 1 inch thick. Using your hands, shape the flattened rice ball into a triangle.

Place half a sheet of nori, shiny side down, and place the triangle rice ball at the center of the top half of the seaweed. Fold the bottom half of the nori over the onigiri and fold the corners down to adhere. Set it aside and repeat with remaining ingredients.