Cut the Spam into 8-10 even slices. Place slices in a resealable plastic bag. Combine oyster sauce, soy sauce, and sugar in a bowl, stirring until sugar completely dissolves. Pour marinade over Spam slices and let marinate for 15 minutes.

Remove Spam from marinade and discard liquid. Heat a skillet over medium heat and cook Spam slices for 3-4 minutes per side until golden brown and slightly crispy.

Place a strip of nori seaweed on your work surface with the shiny side facing down. Position your musubi mold crosswise in the center of the nori strip.

Scoop sushi rice into the musubi mold, filling it about 1 to 1.5 inches high. Press down firmly and evenly with the mold's press. Keep your hands and mold slightly damp to prevent sticking.

If using furikake, sprinkle it over the pressed rice. Place one piece of cooked Spam on top of the rice.

Carefully lift the musubi mold away, leaving a neat rectangle of rice topped with Spam on the nori. Fold one side of the nori over the top, then fold the other side over to create a neat package. Use a small amount of water on your fingertip to help seal the nori if needed.

Serve immediately while warm, or wrap tightly in plastic wrap for later enjoyment.