I buy a lightweight cotton duvet that holds up well with no pilling and has the ties. It's also cool to touch like the cool side of the pillow is. Charter Club brand has many styles. The one I use below.

https://www.macys.com/shop/product/charter-club-damask-1.5-stripe-550-thread-count-100-supima-cotton-duvet-cover-sets-created-for-macys?ID=10255875&CategoryID=25045&swatchColor=Neo%20Natural