I have also used interfacing. The strips were cut 2.5 inches wide, presssed into place on the front and back. The project was placed on longarm, quilted and you would never have known the batting was pieced. Great job!
I will use this method again. I got the idea from the product that is on the market....when I saw it in my LQS, I wondered, if it was nothing more than iron on interfacing. What is your guess?