Prepare the rice: Cook 2 cups of rinsed short-grain white rice according to package directions. Once cooked, transfer to a large bowl and gently fold in rice vinegar and 1 tablespoon sugar while the rice is still warm. Allow to cool slightly until handleable.

Make the Spam glaze: In a small bowl, whisk together soy sauce, brown sugar, and mirin until the sugar dissolves completely.

Cook the Spam: Heat vegetable oil in a large non-stick skillet over medium-high heat. Add the Spam slices and cook for 2 minutes per side until lightly browned.

Glaze the Spam: Pour the soy sauce mixture over the Spam slices and continue cooking for 1-2 minutes per side, until the sauce reduces and creates a caramelized coating on the Spam. Remove from heat and set aside.

Assemble the musubi: Place a strip of nori on a clean work surface. Position your musubi mold (or modified Spam can) in the center of the nori strip.

Layer the ingredients: Wet your hands slightly to prevent sticking, then press about ¼ cup of rice into the mold, creating an even layer. Optionally sprinkle with furikake seasoning. Place a slice of glazed Spam on top of the rice. Add another thin layer of rice if desired.

Form the musubi: Press down firmly with the musubi press or a flat object that fits inside your mold. While pressing down, carefully remove the mold, leaving a neat rectangular stack.

Wrap with nori: Quickly wrap the nori strip around the rice and Spam stack, using a few drops of water on the end of the nori to seal it.

Slice and serve: For easier eating, slice each musubi in half diagonally with a sharp knife. Serve immediately or wrap individually in plastic wrap for later consumption.