Cut the squashes in half lengthwise and remove and discard the seeds. Cut a thin slice from the bottom of each half so they sit flat. Place on a rimmed baking sheet, hollow side down. Place in the oven and pour in ¼ inch of hot water. Roast uncovered for 30 minutes. Leave oven on.
Melt 4 tablespoon of butter in a large skillet that has a lid over medium heat. Add the onion, celery, salt, and pepper and cook until tender, 7-8 minutes. Add the broth, raise the heat, and bring to a boil. Turn off the heat and stir in the stuffing mix until fully combined. Stir in the cranberries, cover the skillet, and let sit for at least 5 minutes.
Carefully remove the squash from the oven, remove the squashes from the pan, and drain off the water. Replace the squash on the pan, hollow side up. Divide the stuffing evenly among the squash cavities, packing it in well. Melt the remaining 4 tablespoons of butter and drizzle over each squash and bake for another 25-35 minutes or until the squash is tender.