I use a product called Block Butler. It looks like batting, but arrives with plastic on both sides, because it is tacky both sides. You have to peel the plastic back and apply it to a wall, door or whatever. It is totally removeable if you need to take it off, and when you peel off the other sheet of plastic, it is nice and tacky to hold your blocks. It won't be as inexpensive as just flannel, but flannel never worked for me. I have no connection with this product, other than I am a very happy user and can recommend it.