Tips: Keeping the chopped pieces small will help the sushi stacks stick together better. If the first one falls apart when turning out, press the next one more firmly. A few pieces often stick to the cup, and you can simply place them back on the stacks. The mashed avocado makes a good glue.
Mix up the vegetables: Besides or in combination with the cucumber, you could use shredded carrots, diced mushroom, cooked sweet potato or asparagus, pickled vegetables of choice, etc. Just make sure to chop the pieces to about ¼-inch cubes and maintain the same ratio of veggies to rice and avocado.
Scaling the recipe: You can make any number of sushi stacks. For each stack, simply plan on 3 jumbo or 4 large, cooked shrimp (frozen and thawed from a pre-cooked bag is fine), ¼ cup cucumber (or mix of veggies), 2 to 3 tablespoons mashed avocado, and ½ cup cooked rice. Then drizzle each stack with 1 teaspoon soy sauce, 2 teaspoons sriracha mayo (or to taste), and a sprinkle of sesame seeds or seasoning blend of choice.
What is sushi rice? Sushi rice is simply short grained rice to which a rice vinegar mixture is added as the rice is finished cooking. Short of that, you can achieve more flavorful rice by adding 1½ teaspoons of kosher salt to the pot when cooking the rice. Though it would taste fine, a long grain rice won’t work in the stacks because the grains don’t stick together sufficiently.
One more thing: I often end up with a small amount of leftover ingredients, which are perfect for a mini sushi bowl snack.