Preheat the oven to 350°F (175°C).

Place grated squash in cheesecloth, and wring out any excess liquid.

In a large bowl, combine the sliced squash and onion with the cheddar cheese, parmesan cheese, sour cream, mayonnaise, garlic powder, salt, and pepper.

Pour the mixture into a 9x13 inch baking dish.

In a separate bowl, mix together the crushed Cheez-It crackers and melted butter. Sprinkle the mixture evenly over the top of the squash mixture.

Bake in the preheated oven for 30-40 minutes, or until the top is golden brown and the squash is tender.

Let the casserole cool for 5-10 minutes before serving.