Preheat oven to 375°. Invert squash in an 11 x 7-in. baking dish. Add water and cover with foil. Bake until tender, 50-60 minutes.
Meanwhile, cook beef, onion and celery over medium heat until no longer pink; drain. Stir in the flour and salt until blended. Add milk. Bring to a boil. Cook and stir for 2 minutes or until thickened and bubbly. Stir in rice.
Transfer squash to a baking sheet. Fill cavity with meat mixture. Bake at 350° for 30 minutes. Remove from oven; serve immediately!