Preheat the oven to 425°F (220°C). Line an 8x8-inch baking dish with parchment paper, leaving overhang for lifting and clean slicing.
Rinse the sushi rice until the water runs mostly clear. Cook the rice with the water in a rice cooker or on the stovetop.
In a small bowl, stir together 1/3 cup rice vinegar, 2 tablespoons sugar, and 1 teaspoon salt until dissolved.
When the rice is finished, transfer to a bowl and gently fold in the vinegar mixture while warm.
Press the seasoned rice firmly into the prepared baking dish into an even layer. Sprinkle with furikake and crushed nori.
In a mixing bowl, combine imitation crab, optional chopped shrimp, 1/2 cup mayo, softened cream cheese, sriracha, soy sauce (if using), and most of the green onions.
Spread the creamy seafood mixture evenly over the rice.
Bake for 10–15 minutes, until warmed through and lightly bubbly. If you’d like a little color, broil for 1–2 minutes watch closely.
While it bakes, cook the frozen shrimp tempura until very crisp (air fryer or oven works best).
Remove the bake from the oven and immediately pile the crispy shrimp tempura on top.
Stir together the spicy mayo drizzle (1/3 cup mayo + 1–2 tablespoons sriracha) and drizzle over the top. Add eel sauce drizzle.
Finish with sesame seeds, tobiko/masago (if using), nori flakes (optional), and the reserved green onions.
Let rest 5 minutes, then lift out using parchment and cut into squares. Serve with nori sheets and soy sauce.