Marinate shrimp in sriracha, 2 tbsp mayo, sesame oil, and soy sauce for 10 minutes, then sear in a hot oiled skillet for 2 minutes per side.
Season hot cooked rice with rice vinegar, sugar, and salt, folding gently until glossy. Cool to just warm.
Toss diced avocado with lime juice. Mix remaining mayo with a teaspoon of sriracha for the drizzle.
Layer rice, cucumber, and avocado inside a lightly dampened round mold on a plate, pressing each layer firmly.
Top with seared shrimp, lift the mold straight up, then drizzle with sriracha mayo and finish with sesame seeds and nori strips.