Retread
Sounds good - but is coverless duvet not just a fancy name for a quilt? 😄
We sleep happily under an all cotton bed cover from IKEA and a top sheet. I have them in 3 different colours and have had them for years (cost was about £40 each).
As Luckygirl said, you don't need a top sheet.
Why call them "coverless duvets" and not "quilts"?
To me, quilts are colourful bedspreads, usually-hand made (or made on a domestic sewing machine in a home, not a factory) and quilted all over in patterns. They are not very thick, and are meant to lie over the top of the bedclothes and hang down round the edges (or to hang decoratively on the wall) In cultures where quilts are the traditionable bedding, they usually use several on top of one another, depending on the local weather.
Eiderdowns used to lie on top of the bedclothes, too, but they stopped at the edge of the bed. The best were filled with eiderduck down in a down-proof inner lining, next expensive with goose down or duck down, then mixed feathers and down, the cheapest being more feathers than down and correspondingly heavy. The visible side of the covering fabric was silk or silk substitutes - rayon, nylon and other manmade fabrics - and the underside of a plainer fabric, all quilted through in geometric and decorative stitching.
Duvets have been used for centuries. The "feather beds" of the past were often used on top of the sleeper as well as underneath. On the continent the tradition continued, but in the Uk they only became popular in about the 1970's. The first ones were filled with all down, with feathers and down, or just with feathers. The ones I slept under on a visit to Germany on the late 50's had an immaculate white cotton loose cover. It was incredibly comfortable, and very light, so must have been all down.
When manmade fillings began to replace feathers and down for pillows, they also began to be used for duvets, as they were lighter, and more washable, except that with wear and washing the earlier fillings tended to stick together in lumps. Plastic drinks bottles can be recycled into fibres which don't clump together, which means the whole duvet can be washed and tumble dried and stay loose and airy. This led naturally on to making the duvet of an attractive fabric which didn't need a cover over it - and didn't need a contortionist to put a washed cover back on.
Many coverless duvets are made by people who previously made plain "duvets" which were in white cotton or a similar fabric and were expected to be put into a duvet cover. This is why they are called "coverless duvets" and not "quilts".