A duvet cover is just a big pillow case and easy to make. The thing is getting material that is wide enough to make it.

I suggest you look at sheets. If you can find a pattern you like it would be easy to make a duvet cover and even drapes out of flat sheets.

The thing about sheets is they are already hemmed. You need 2 flat sheets to make a duvet cover. Measure your duvet buy the sheets that most closely matches the cover. Leave the top hem alone and cut any excess off the bottom and sides. Make sure you leave enough on the top to overlap the seams. Now sew all three sides leaving the top open. Then sew velcro on to the hems to close the envelope

Thanks - I'll start looking