Slice spam into 8 horizontal slices. Fry in a large skillet until lightly browned, about 2 minutes per side. Remove from skillet. Add tamari, mirin, and brown sugar. Heat, stirring, until sugar melts and it starts to make a thick glaze. Add the spam slices back in, flip and coat with glaze.

Place a thick layer of rice in the bottom of the musubi mold. Top with glazed spam and release from the mold. Wrap with a nori strip.