A white comforter is a problem! White always gets dirty. At least in my house. I made my own duvet cover to 1 - save money and 2 - protect my Hungarian goose down comforter.
I measured the length of the comforter - roughly 2 1/4 yards. I then purchased about 4 1/2 yards of two different fabrics (front and back). I cut the fabric in half and created a seam on the front and back of my duvet.
I left a bit extra from the front to sew over to the back side. I also top stitched the front and back around the seams for a finished edge.
I hemmed the open ends at the bottom (allowing an opening for putting in and taking out the duvet). I sewed 8-9 snaps along the bottom to close the opening.