In a medium bowl, toss the shrimp with olive oil, garlic powder, paprika, salt, and black pepper. Set aside to marinate for 15 minutes.

Rinse the sushi rice under cold water until the water runs clear. In a medium saucepan, combine the rice and water. Bring to a boil, then reduce heat to low, cover, and simmer for 18-20 minutes until the rice is tender. Remove from heat and let it sit covered for 10 minutes.

In a small bowl, mix rice vinegar, sugar, and sesame oil until dissolved. Stir this mixture into the cooked rice, fluffing it gently with a fork. Allow the rice to cool to room temperature.

While the rice cools, heat a skillet over medium-high heat. Add the marinated shrimp and cook for 2-3 minutes on each side until they turn pink and are cooked through. Remove from heat.

To assemble the stacks, use a round mold or a small cup. Layer the ingredients starting with a spoonful of sushi rice, followed by a layer of shrimp, avocado slices, cucumber, and a drizzle of spicy mayo (mix mayonnaise and sriracha together). Repeat the layers until the mold is full.

Carefully remove the mold and garnish with chopped green onions and nori strips if desired. Serve immediately.