What would work best?
 

My DSIL has a quilt top made from polyester & other types of fabrics. The seams are whipped together making it not lie flat & it is very heavy. My question is-Can I use spray adhesive on the top & lay a flannel backing over it?? Will it stick enough to quilt it with straight lines? Thanks for your answers!

I wouldn't worry about batting. I would use a flannel backing and put it together envelope style. Then I would hand or machine tie it every six inches or so. That is the way I make many of my Linus quilts if the fabrics are not all of the same weight or fiber.