Fold the rice vinegar, sugar, and salt into the hot cooked rice. Cover until ready to use.
In a very small bowl, combine the tamari and the lime juice and set aside to use as a dipping sauce.
Make the spicy mayo: In a small bowl, stir together the mayonnaise and the sriracha.
Serve the nori squares, rice, cucumber, mango, avocado, pickled ginger, sesame seeds, and desired extras, on a platter to assemble as you eat, mixing and matching fillings as you like. Serve with the tamari-lime dipping sauce and spicy mayo.