Cut Spam into 10 slices. Fry until slightly crispy. Remove and drain on plate lined with paper towels. In another pan, combine soy sauce, mirin and sugar. Bring to a boil over medium-high heat, then reduce to low. Add Spam slices, coating them in the mixture. When mixture has thickened, remove Spam from pan.
Lay a sheet of nori lengthwise on a clean surface. Moisten lower half of musubi maker (see Note), and place on lower third of nori. Fill musubi maker with rice and press flat until the rice is 3/4-inch high. Sprinkle rice with furikake. Top with slice of Spam. Remove musubi maker and keep in a bowl of warm water to keep it clean and moist.
Starting at the end towards you, fold nori over Spam and rice stack, and keep rolling until completely wrapped in the nori. Slightly dampen the end of the nori to seal it. Repeat with the other nine Spam slices, making sure to rinse off musubi maker after each use to prevent it from getting too sticky.
You can also use an empty Spam can that has been opened on both sides for the musubi mold, using your hands (or a piece of Spam) to press down on the rice.