If you don't have luck with homemade cornstarch mixture, I'm wondering if you could use a little white washable glue greatly diluted and sprayed or brushed onto the your flannel. Since it washes out, it might be just enough to stabilize the fabric so it won't distort when you sew. Sharon Schamber talks about the washable white glue in her binding tutorials. I used her technique and was amazed at how much the glue helped. My project now looks professionally finished.