What about a duvet cover? I've made one of those and depending on how fancy you get, it's basically a pillowcase. I made one about a year ago and the hardest part was managing all the fabric. In the summer, we leave it empty and in cool weather I have a down comforter I put in it.

I've made both and I can attest to the fact that making a duvet cover is much simpler. Bonus points to the duvet cover because you can remove it to wash easily and make more than one to change the look of the bed. Easy to sew with flat bed sheets and close with velcro.

For making one from scratch you can use polyester batting that you buy at a place like Joanne fabrics or a blanket (use more than one layer for a richer look). You will then have to attach all the layers to keep the comforter intact. The hard way is to hand stitch a beautiful pattern like you see on handmade quilts - this is challenging to do well. A slighter easier method is to use a sewing machine to sew the layers together similar to what you see on commercially available quilts. Last you can take small single prick stitches to keep the layers together - I can't remember what these are called but they can create a really pretty look if done well.

Sky

I personally like to match my batting to my fabric which gives a better foundation for future cleaning. Silk for silk, cotton for cotton or wool if you want the warmth, and poly if I use a synthetic.

You can get a spray on glue (at a quilting store) that just temporarily tacks it to the outside layers and then you can do a machine quilting on it, but don't try if you don't have a quilting foot. The layers will walk on you without it and it will look like goober on one side. (Screws up the tension). But it's not hard to do :)

Buy a duvet which is similar to (but not the same as) a comforter but usually covered in white cotton fabric and filled with down, altho you can get synthetic ones also. Then make the duvet cover as previously suggested.

Duvets can be expensive but IMO feel better than comforters and can last a long time if kept covered.