Margo wrote:
Lots of things will work! Flannel, fleece, felt, or even cotton batting! If you have something you think might work, give it a try!
The advantage of having a backing for the top fabric is so that you can stab pins into it, but it's not necessary.
Just be aware that the larger (and heavier) the piece you want to adhere, the more likely you will need to pin it somehow to the fabric.
I used fleece, but it has a definite stretch. I used a nonwoven fusible interfacing on the back of the fleece (the second time that I redid the boards because the first time the fabric stretched). I don't know what type of glue I used, but it wasn't spray. Love my fleece covered lightweight insulation board. Eliza Pryor Lucy said to use a neutral and that was what I had at the time and I love it. In a class with Hollis Chatelaine, she told us to turn the batting to the back because the white grabbed her eye and that was what she saw, not the quilt.
Regardless of what you do, it is so amazing when you look at your project upright and from a distance. You have a better perspective.