Combine soy sauce, sugar, and mirin in a small bowl. Slice and fry Spam over medium heat until a nice golden brown. Lightly brush soy sauce, sugar, and mirin mixture onto cooked Spam.
Place sandwich bag into empty Spam can.
Fry kimchi and rice in butter over medium heat.
Compress 1/2 cup kimchi fried rice into sandwich bag with a spoon. Place a slice of Spam atop fried rice.
Compress layers firmly with a spoon.
Center your mold and flip can over onto a horizontal strip of nori. Seal nori strip with a touch of water.