Cut squash in half lengthwise and brush lightly with oil (seeds are easier to remove post-roasting)
Bake at 375°F for 35 minutes. Let cool.
Scoop out seeds. Scoop squash meat out of skins. Puree for 15 seconds until smooth in batches until all squash is pureed.
Make mac & cheese:
Preheat oven to 325°F
Melt butter in 4 qt. pot, stir in flour cook for 1 minute, stirring occasionally
Remove from heat, stir in milk. Place pot back on low heat, allow milk to warm. Stir occasionally. Add cheese and mix until melted. Stir in salt and pepper. Stir in squash puree, 2 cups at a time.
Allow sauce to thicken on low heat while pasta is being prepared.
Bring 3 quarts of water, salted with 1/2 cup of salt, to a boil. Add macaroni and cook for 8 minutes.
Combine pasta and sauce in a large bowl. Divide mixture and add to 2 - 20x12x4 pans.
Cover with foil and bake for 25 minutes. Remove foil, bake for 10 more minutes until browned.