In a small bowl, combine the cooked sushi rice with rice vinegar and sea salt, fluffing gently with a fork to incorporate.
Drain the skipjack tuna thoroughly and place it in a separate bowl.
Mix the tuna with avocado oil mayonnaise, sriracha, and lime juice until well combined.
Slice the avocado into thin wedges and julienne the cucumber into matchstick-sized pieces.
Place one nori sheet on a bamboo rolling mat or a clean flat surface, shiny side down.
Spread half of the seasoned rice evenly over the bottom two-thirds of the nori sheet.
Layer half of the tuna mixture, avocado wedges, and cucumber matchsticks across the center of the rice.
Carefully roll the nori from the bottom up, using a tiny bit of water to seal the edge of the sheet.
Repeat with the second nori sheet and remaining ingredients.
Use a very sharp, damp knife to slice each roll into 6-8 pieces and serve immediately.