That was my first thought too, especially since flannel is heavier to begin with.

I generally use inexpensive muslin for foundations. I do prewash it before cutting, cut my squares ( or rectangles) a couple inches larger than I want my blocks - that gives me plenty of room to play with my scraps- then when I get all my blocks finished I trim them to all the same size.
Sometimes I find I have many scraps of white, or a light fabric that are long strips maybe cut from quilt backings and I use those for foundations then it’s all scraps in the quilt. :)

Bought that sediment stuff...works great...roll will last 2 lifetimes! But before that I used tissue paper (gift wrap), or telephone books- don't see them anymore-and now I get perspectives in the mail, same thin paper so I can use them, better than trying to read/understand them. I used fabric once, to heavy, even though it was a low grade muslin...paper is crisper to work with..jmho

I use old bed sheets which I have cut into squares slightly larger than the finished size of the blocks. My wife really likes to buy new sheets every few months, so I have a good supply of fabric to use for foundations. It's a shame, sometimes, that the foundations don't show in the finished quilts; they can really be quite colorful.

I also re-purpose bed sheets.

I have done several string quilts. Most of them with uneven sized strings. I have used several different foundations but the one I like the best is phone book pages. However, I do take the paper off as soon as I have the block squared up. I am working on one for 2 years, as I tire of the same thing. I have 32 done but need over 60. Am going to a quilt retreat in Feb and one in April and hoping to at least finish up making the blocks then. But I so enjoy looking at all the different colors and remembering other quilts I have made. They become addictive.

I am going to look into this option. I like the idea that it can stay in the quilt and is economical. Thanks for the heads up.

I made a quilt with small 6" squares while staying in my RV. I used dryer sheets that my friend saved for me. Worked out pretty good and used my leftovers. Saved me because I had finished my quilt top and was dying to keep making more!

I have used Pattern Ease or Easy Patern for foundation and it makes a lighter weight quilt than muslin. It does not need to be removed. These products are used for tracing patterns when there are multiple sizes on a pattern. It is nonwoven thin fabric similar to an interfacing. I love string quilts and have made tons of them. Have fun!

I use a web mesh stabilizer. I have rolls of it that I use for machine embroidery. It is soft, pliable, washes well and keeps it's shape. I especially like it because it can stay on the block and I do not have to remove it. I use it for all my embroidery quilt blocks. I get mine from a shop in Conroe, TX, but it can also be purchased at almost any shop that sells stabilizer of any kind.