Combine the diced tuna, sesame oil, hot sauce, soy sauce, ginger, green onion, and rice vinegar in a bowl. Adjust the sriracha and soy sauce to taste. Place in the refrigerator to marinate while you prepare the vegetables. Meanwhile, peel the cucumbers and daikon and cut into small dice. Place in a bowl. Cut the avocado into small dice and add to the bowl. Stir in the cilantro, sesame seeds, and vinegar and coat the vegetables well. To assemble, use a 2-inch tall by 2-inch diameter cylinder (a small tomato sauce can with both ends removed works well). Place the cylinder on a small appetizer plate. Fill the can halfway with the vegetable mixture then top with the tuna mixture. Press the tuna firmly to pack the ingredients. Lift the cylinder slowly while pressing down on the tuna with the back of the spoon to slide the tower out on to the plate. Repeat with remaining ingredients. Garnish the ahi tuna towers with pickled ginger and wasabi paste and a drizzle of sriracha if desired.