I think I understand the problem. When you install that bag, the back/bottom tilts against the head tube and brake cable? If that's the issue, I solved a similar problem by just getting a piece of flat aluminum piece, 4"-5" long and maybe a 1/2" wide, bending it into a Z shape (with 90 degree angles), and attaching it to the stem through the handlebar bolt (drilled a hole). So one end (short vertical length) bolts into the stem, then a horizontal extension of the appropriate length to reach the bag, and then a vertical length that will push against the back of the bag to keep it vertical and off the head tube. Hard to explain, but super simple.
Or you can look up "handlebar extender" on amazon. This is a short length of handlebar attached to your handlebar with a couple of clamps. They should do the trick, but is more involved that you may want.