To keep shrimp straight, make several shallow crosswise nicks on their inner curved sides with a knife, being careful not to cut through shrimp. Put in a bowl.
Mix together salt, sugar, pepper, and garlic powder. Sprinkle onto shrimp and toss gently to coat.
Cut green onions into lengths roughly the same length of the shrimp and put in a small bowl. In another small bowl, mix cornstarch with 3 tbsp. water to make a thin paste. Stack 12 spring-roll wrappers, cut them in half diagonally into triangles, and cover with a damp kitchen towel to keep from drying out.
Pour enough oil into a 6-qt. pot to come 2 in. up sides and insert a deep-fry thermometer. Heat over medium-high heat to 375°.
Meanwhile, working with 1 triangular wrapper at a time, lay 1 piece of green onion and 1 shrimp on a narrower corner of wrapper, with tail of shrimp extending over edge. Brush opposite corner with cornstarch slurry. Tightly roll wrapper once over shrimp and green onion. Fold the middle point of the triangle over shrimp and green onion, then finish rolling to seal. Repeat with remaining wrappers, shrimp, and green onions.
Fry shrimp 5 or 6 at a time until golden brown, about 5 minutes. With chopsticks or long tongs, transfer to rack set on a baking sheet. Let drain briefly, then serve hot, with Ginger Fish Sauce.
Make ahead: Through step 5, up to 6 hours, chilled (pat dry before frying), or 1 month frozen (fry straight out of freezer). You can also fry shrimp the day before and reheat 15 minutes in a 350° oven (does not work for shrimp fried from frozen--they toughen).