Anyone have/had something like this? The Horseware Amigo All-In-One type of sheet and blanket. It has no seams and it is all one piece including the full neck. It’s available as a sheet (no fill) or with various fills (medium or heavy) :
https://shop.horseware.com/product/amigo-bravo-12-all-in-one-turnout-0g-lite/