Make the sauce: In a small bowl, mix together soy sauce, mirin, and sugar until combined. Set aside.
Slice the Spam: Remove from the can and slice into 8 even rectangular pieces lengthwise.
Cook the Spam: Heat a pan over medium heat. Cook Spam slices until golden and crispy, about 2-3 minutes per side. Lower heat, pour in sauce, and simmer until it thickens into a glossy glaze. Remove from heat.
Make the egg layer: Whisk eggs with salt and pepper. Cook in a lightly oiled pan over medium-low heat into a thin, even sheet. Use the empty Spam can to cut out 8 rectangular pieces.
Assemble: Line the Spam can with plastic wrap. Press a couple tablespoons of rice in firmly. Sprinkle with furikake. Add one egg piece and one glazed Spam slice. Lift out using the plastic wrap.
Wrap with nori: Wrap each musubi tightly with a strip of nori. Serve immediately for best texture.
