Shin bang is usually caused by the presence of space/gap between the shin and the liner tongue/front of the boot.
Why do a skier need to get to the front of the boot?
The answer may surprise you.
In this case the shin bang is in one boot only, after 3 years or so of use. I bet my brand new Smith helmet the cause is packed tongue and it is also in the right boot.
So, to take up that space you can indeed use a Booster strap, or if you happen not to be in love with elastic stuff-the pads i posted above.
The Sidas ones work perfectly-tried and tested. And relatively cheap.