Preheat the oven to 350 degrees F (175 degrees C).
Pour 5 cups water into a saucepan. Cover and bring to a boil over high heat. Add macaroni and boil until tender yet firm to the bite, about 8 minutes.
Meanwhile, melt butter in a saucepan over medium heat. Stir in flour until a smooth paste is formed. Slowly pour in milk, stirring to incorporate. Add 1/2 cup Cheddar cheese, sour cream, and Cajun seasoning. Drain macaroni and stir into the sauce. Add frozen shrimp and stir to combine.
Pour macaroni mixture into an oval au gratin dish. Top with panko bread crumbs and 2 tablespoons Cheddar cheese.
Bake in the preheated oven until bubbly and golden, about 20 minutes.
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