I have a giant king pure linen duvet cover that I place a silk stuffed comforter in. It's like covering up with butterfly wings of warmth. In summer we put a thin cotton quilt in the duvet cover. Hangs to the bottom of the highest mattress. More on reddit.com

I do this; it’s the only way to get better coverage. I also buy King sheets for the little bit larger top sheet and to have extra depth for my featherbed. More on reddit.com

This is why I buy king sized duvet for our queen bed. It feels like the size meant for beds just don’t look good to me. More on reddit.com

I know White Lotus Home has an all-cotton fill comforter and I think they do custom sizing but it tends to be really pricey and is not washable. Cotton won't be super-fluffy like down; it will have the slightest bit of puff to it but it will eventually compact and firm up over time. For a pretty decent budget option I think Costco has one that's 100% cotton fill (though I don't think it's oversized) and one can usually find some good options if you search google with 'cotton fill comforter'. Good luck! :) More on reddit.com