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.
Compress hot rice into sandwich bag with a spoon.
Spread a layer of sriracha mayo across the top of rice.
Place a slice of Spam atop the layer of mayo.
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.