You might also try putting some darts into the back to shape it -- so it tapers in down to the waist, then back out again over the upper hips. Pin them in, try to get them even on each side, chalk along the pin lines, baste in place and try on again, then sew. If it already has rear side seams (not just underarm and center back seams) you can take it in there, along the same taper. Kilt jackets look better when they hug the body -- most American sport coats are cut to hang more loosely.