Duvet covers are more convenient to wash than washing a whole comforter but they are extremely annoying when you have to stuff the duvet inside, or when your duvet migrates around inside and doesn't stay in place.
I finally put some thought into an easy fix. It only took about an hour and I already had everything on-hand.
WHAT YOU NEED:
4 large eyelets (attach to corners of duvet)
4 sets of Velcro* pieces (attach to corners of duvet cover)
*I had 1"-wide Velcro and cut it down the center to be 1/2" so that it would fit through the eyelets.
The idea is that the Velcro hooks around the eyelets to hold the corners of your duvet in place so that it can't move around. The Velcro makes it easy to center your duvet inside your cover in case it is slightly smaller than your cover (mine is 2" smaller around all sides).
I used a fabric marker to mark the center hole of the eyelets on each corner of my duvet. Give your eyelet a little space from the edge so it has enough fabric to hold onto.
Sew a circle around your dot, about 1/8" away from the edge of the marking.
Cut around your marking, keeping your stitching intact.
Add hardware and give it a good hammering.
Make sure you hammer hard enough that it has a good curl around the edge to hold all the pieces together.
Figure out how long you need your Velcro. You want your hook & loop pieces to overlap at least an inch. I did my loop piece about 5-inches and my hook piece about 3-inches (because my duvet was about 2-inches smaller around each side than my duvet cover).
Make sure you duvet cover is inside out! Attach Velcro to each corner of your cover like so. Go back and forth a few times to ensure it's anchored well.
With your duvet cover still inside out, lay it down flat with your duvet on top.
Attach velcro through eyelets and you're ready to turn your cover right side out through the opening of your cover. Give it a good shake and pull on each corner to help everything lay flat inside.