In a small bowl, whisk the soy sauce, sugar and the mirin. Set aside.
Using a skillet over medium heat, add the oil and fry the Spam slices until evenly browned and crispy, about 2 to 3 minutes on each side. Add the soy sauce mixture and turn the Spam slices until evenly coated in the glaze. Remove from the heat and transfer the Spam slices and the glaze to a plate.
To assemble the spam musubi: Start by placing a strip of nori, shiny side down, on a cutting board or clean work surface. Place a Spam musubi mold over it, in the center, then place a slice of Spam into the mold. Sprinkle a little furikake over the Spam, then fill the mold with some rice. Press the rice down with the musubi maker press or use your hands until it is about 3/4- to 1-inch thick, adding more rice as needed.
Use the press to hold the rice down with one hand and pull the mold upward to unmold the musubi with your other hand. Wrap the nori around the Spam-rice stack, bringing both ends of the strip to the middle, folding one over the other, and flipping it over so the seam is down and the Spam is facing up. Repeat with remaining ingredients. Enjoy!