@Doublegreen
I don't think cork is going to solve your issue with a complete top gap especially when you say gap moves to the back calf, meaning you can bang it back and forth. As I mentioned previously, booster strap is probably what you need to try to keep the top liner against your shin and give you top support.
The cork I feel is more when the liner is already against your shin; but there is a contact issue due to shape of your shin or sharp boniness going through to the plastic causing pain
After the boosterstraps are installed, the issue now is more about the gap between the liner and the shell. the booster strap itself can also help fill that, or you may fiddle with the top buckle or need some aftermarket padding to take up the gap
Anyway it's your boot and foot, just my 2 cents.
you can go ahead and try a couple tubes especially to take up the lower ankle gap issue you say you're also having.