Preheat the oven to 350 degrees F (175 degrees C). Grease an oval au gratin dish.

Bring a large pot of lightly salted water to a boil. Cook elbow macaroni in the boiling water, stirring occasionally, until tender yet firm to the bite, about 8 minutes. Drain.

Melt butter in a saucepan over medium heat. Stir in flour until a smooth paste is formed. Slowly pour in milk, whisking to incorporate. Stir in 1/2 cup Cheddar, sour cream, and Cajun seasoning. Stir in drained macaroni, then stir in frozen shrimp until well combined. Pour into the prepared dish, then sprinkle panko and 2 tablespoons Cheddar over top.

Bake in the preheated oven until bubbly and golden, about 20 minutes.