If you are thinking of using new flannel, wash and dry more than once, because it will shrink a lot!!!!! The cut edges will probably not be 'straight' if you fold it along the selvedge. That's just the nature of the beast. Gently used flannel sheets are a good option.
Many people will use a piece of fleece for backing and leave out the batting. I made a lap quilt for a friend's 100 year old mother, using a pretty cotton print and a fleece throw for backing. I quilted it on a longarm and bound it. My friend's mother is 102, the lap quilt is in constant use and is holding up well.
I don't know about polyester double knit--I knew it as Fortrell in the day. (I think that was actually a brand name.) For one thing, it doesn't breath at all, for another, it is extremely flammable and will melt if subjected to high heat. My sister made car blankets for all her grandkids and backed them with Fortrell. I think the cars died before the blankets LOL!