Thus far, I've only made duvet covers to go under a decorative spread I can pull on or off the bed. High thread-count sheets work the best because of their softness and light weight. You will have to be careful that the matelasse fabric you use is not too stiff or heavy.

I like the design of a duvet cover my sister brought me from Finland. It has an opening at the top of each side so that the person making the bed can reach in and pull the duvet up and into the cover when making the bed. I finish these openings last after creating french seams on the rest of the three closed sides.

On the last two covers I've made, I've cut the pieces out so that I can use the hemmed ends of the sheets for a ready-made finish on the open end that remains at the foot of the bed.

I make comforter covers from sheets because DH and I don't like wrestling with a top sheet, and it's easier to make the bed. I haven't tried making one with fabric of different weights, but whatever you use, you should launder your fabrics first to pre-shrink and to correct any shifting (ie, you may have to square it up before sewing). I have found matelasse duvets online and they are made with a smooth underside, so maybe two different weights would work fine together. I've attached a link below.
I agree with Corgi's advice about the weight of your fabric, though. I've used flannel sheets and even those will make a comforter quite heavy. Good luck and let us know how it turns out.

Here is a link that might be useful: matelasse duvet covers

Noinwi, do you wash your sheets before making the duvet cover? I was at a loss because I couldn't find a duvet cover that didn't cost a fortune and that was light. My down comforter was extra large king and the duvet covers were all smaller. Also, I had a perplexing time trying to find flat sheets, all they seem to sell these days are sets. Anyway I found flat sheets on sale that I plan on using to make a cover when I figure out how to do it. In the meantime I found a white duvet cover, lightly embroidered, out of cotton material that fits the comforter pretty well (for $20) on closeout at Burlington. This will work well under my spread or quilt until and if I find shams to match. How do you keep the comforter from slipping around inside? I bought some large safety pins and pinned it to the seems inside. I'm thinking I can sew some tape ties on the inside edge although not too wild about sewing these onto the comforter.

Terra, yes I do wash my sheets and/or fabric before sewing. They usually do some (diagonal)shifting when laundered and you may have to trim them a little to square them up before sewing. That way, your cover won't twist on you after laundering the way new covers do.
I agree it's hard to find separate sheets these days. They are either really cheap(quality)like the ones from "Wally World", or really expensive. Some fabric stores have extra wide fabrics(muslins, flannels), but are limited on colors. I try to find mid-range sheet sets and match fabric to the top sheet. It doesn't matter(to me)if there are seams on the top side of the cover. You could also buy two sets and use the top sheets for a reversible cover(best done during a big sale).
If you sew small loops(from fabric or twill tape)on the corners of your comforter they hardly show, then you can sew ties to the inside corners of your cover to keep the comforter from shifting around.
As far as making the cover, just think of it as a large pillow case sewn on three sides, with a hemmed edge on the open end. I usually make buttonholes along the opening, but sometimes when I'm rushed or tired, I'll sew fabric or tape ties(at least three pairs)to close the end(which I place at the foot of the bed).
Good luck. I hope this helps.