Help with a Duvet Cover

I want to make a duvet cover for my daughter's room.  I wanted to make a quilt, but started feeling very overwhelmed so that perhaps a duvet cover would be much easier.  Does anyone know how much fabric I would need to do a twin size cover?  If I used a different fabric on each side?

Thanks

You should probably measure the duvet. I have two different queen size duvets and they are different sizes.