In a medium bowl, toss the shrimp with olive oil, garlic powder, paprika, cayenne pepper, salt, and pepper. Let marinate for 15 minutes.
Heat a skillet over medium-high heat. Add the marinated shrimp and cook for 2-3 minutes on each side until they are pink and opaque. Remove from heat and set aside.
In a separate bowl, mix the cooked sushi rice with rice vinegar and sugar until well combined.
To assemble the stacks, use a round mold or a cup. Start with a layer of sushi rice, pressing down gently.
Add a layer of sliced avocado, followed by a layer of shrimp, and then a layer of cucumber. Repeat the layers until you reach the top of the mold.
Carefully remove the mold and drizzle the spicy mayo (mix mayonnaise and sriracha) over the top. Sprinkle with sesame seeds and garnish with chopped green onions.
Serve immediately with nori strips on the side, if desired.