Place the rice in a fine mesh colander & rinse it well with hot water until the water runs clear. Once water runs clear, drain any excess water from the rice, then transfer it to a rice pot. Following the ratios provided on the rice package directions, add water to the rice pot. Cover & cook. Once the rice is done, let it steam & rest for 5-10 minutes.
Combine all listed ingredients in a small bowl, whisking to combine. Microwave for 30 seconds – 1 minute to dissolve the sugar. Stir to combine then set aside to cool slightly. (Alternatively, you can combine all ingredients in a small saucepan over medium-low heat & cook, stirring occasionally, until the sugar is dissolved, 4-5 minutes.)
Place a large nonstick skillet over medium heat. Very lightly spritz with nonstick cooking spray. Arrange the sliced Spam in a single layer in the skillet. Cook 4-5 minutes, until lightly browned. Flip. Reduce heat to medium-low. Spoon 1 Tbsp of the prepared teriyaki sauce over each slice of Spam. Cook 2-3 minutes longer. Repeat flipping & spooning sauce on the Spam 1-2 more times, until the Spam is as browned & saucy as you like & the teriyaki sauce is reduced to a thick glaze.
Place a large piece of plastic wrap or wax paper on your work surface. Grab a piece of sushi nori. Sushi nori has 2 distinct sides – one is rough & textured while the other is shiny & smoother – & is ever-so-slightly rectangular. Place a piece of sushi nori on top of the plastic wrap/wax paper such that its shiny & smooth side is facing down & one of its slightly longer sides is closest to you. Place the outer box of the musubi mold on the center of the nori such that its long edge runs parallel with the longer sides of the nori.
Use a rice paddle to scrape a small amount of rice off the top of the rice pot. Gently add the rice to the musubi mold, filling it almost entirely full. As you fill the mold, use the edge of the rice paddle to gently pat the rice down into a uniform level but, again, avoid pressing down on the rice too much. Once the mold is filled with rice, sprinkle a generous amount of furikake seasoning over top, about 1-2 tablespoons. Arrange two slices of the teriyaki Spam side by side in the musubi mold.
Place the musubi mold lid on top of the Spam. Press down on the lid firmly, starting at the center & working your way to the outer edge. Once you reach the outer edge, press down on the lid firmly with your thumbs & pointer fingers while using your pinky fingers to simultaneously pull the musubi mold box up. Set the box to the side, then remove the musubi mold top off of the Spam & set aside. Gently-yet-firmly pull the side of the nori sheet facing you up & tautly fold it over the Spam – the nori should stick to the Spam. Brush a little water over the opposite edge of nori, then tautly roll the musubi away from you, sealing the musubi shut. The musubi should be seam side down at this point.