When making your straps, make sure they will be long enough to cover length of the bunchy-ness. I cut two 3.5" X 4.5" rectangles, folded them in half, right sides together, so you have two 1.75" X 4.5" rectangles, and sew edges with a 1/8" seam leaving one short side un-sewn.
Sew a straight line to attach the strap to the seam then flip strap and sew another straight line to hide the unfinished edge.
Now sew your buttonhole on the end of the strap
Try it on and let me know how it turned out!