No to glue and stiffeners and I haven't mastered the technique of posting photos. Sorry.
One of the quilts, I stitched a diagonal 1" grid over the entire sandwich and then I appliqued. The applique was extensive. The borders were added later by adding to the original rectangle. At one point, I thought the bubble was caused by the border. I removed the borders (3) with mitered corners and replaced to no avail. I have often thought of taking all the borders off and make sure the center rectangle lays flat and then reattach the borders.
It hangs in the hall and it seems everytime I pass it, I run my hand down over it to flatten. Uurrgghhhhhh!