Whisk ¼ cup soy sauce, ¼ cup packed brown sugar, and 2 tablespoons mirin if using together in a small bowl.

Remove the SPAM block from the can and lay it on its bigger flat side. Cut lengthwise into 8 (about ⅜-inch-thick) slices. Heat 2 teaspoons neutral oil in a large frying pan over medium heat until shimmering. Add the SPAM slices in a single layer and fry until browned and crisp, 2 to 3 minutes per side. Transfer to a paper towel-lined plate.

Wipe out the pan with a paper towel. Add the soy sauce mixture and bring to a boil over medium heat. Reduce the heat to low, return the SPAM slices to the pan, and cook until the sauce thickens and coats the slices, about 1 minute per side. Remove the pan from the heat.

Assemble the musubis one at a time: Lay 1 nori strip on a work surface or cutting board with a shorter side closer to you. If using a musubi mold, place it perpendicular across the middle of the nori strip. Fill the mold with ½ to scant ⅔ cup of the warm cooked rice, then press it firmly and evenly into the mold. If using the empty SPAM can, fill it with rice, then press it firmly and evenly into the bottom of the can. Flip it out of the can and place the rice perpendicular across the middle of the nori strip.

Sprinkle the rice with ¼ teaspoon of the furikake if using. Top with a slice of SPAM. Remove the musubi mold if needed. Wrap the rice and SPAM tightly with the nori strip, sealing the edges with a dab of water if needed. Eat immediately or wrap each musubi in plastic wrap and eat within 2 hours.