Rinse 3 cups of short grain rice under cold water until the water runs mostly clear, then drain well. Cook the rice in a rice cooker or on the stovetop according to the package directions, using slightly less water than you would for fluffy rice, since musubi rice should be sticky.
Once cooked, let the rice sit covered for 10 minutes, then fluff gently with a rice paddle or fork. Set the rice aside, keeping it covered with a damp towel so it stays warm and moist while you prepare the rest of the ingredients.
In a small saucepan, whisk together 3 tablespoons of soy sauce, 2 tablespoons of brown sugar, and 1 tablespoon of oyster sauce. Place the saucepan over medium low heat and stir occasionally until the sugar has fully dissolved, about 2 minutes.
Remove the glaze from the heat once it is smooth and slightly thickened, then set it aside near your stovetop.
Remove the Spam from the can and slice it into 8 even pieces, about a quarter inch thick. Heat 1 tablespoon of neutral oil in a nonstick skillet over medium heat.
Add the Spam slices in a single layer, being careful not to crowd the pan, and fry for 2 to 3 minutes per side until golden brown. Transfer the fried Spam to a plate lined with paper towel to remove excess oil.
Return the fried Spam slices to the skillet over low heat. Pour the prepared glaze over the Spam and gently toss or flip each slice to coat evenly.
Let the Spam simmer in the glaze for 1 to 2 minutes, until the sauce thickens and clings to the meat, then turn off the heat.
Lay a half sheet of nori shiny side down on a clean cutting board or plate. Place the musubi mold in the center of the nori and fill it about halfway with warm rice, pressing down firmly and evenly with damp fingers or the mold press.
Sprinkle a pinch of furikake rice seasoning over the pressed rice if using. Lay one glazed slice of Spam on top of the rice, then remove the mold by lifting it straight up.
Fold the long ends of the nori strip up and over the top of the Spam and rice, overlapping them slightly. Dab a little water on the overlapping edge of the nori to help it stick to itself and seal the musubi closed.
Wrap each finished musubi tightly in plastic wrap to hold its shape and keep the nori from getting soggy before serving. Repeat with the remaining rice, Spam, and nori until all 8 musubi are assembled.