Adjusting the back strap

By changing the position of the back strap, you can have more fit to the boot. 

The best posture for the strap is when it follows the grooves on to the shell  (where the ball head rivets are) and the strap rests above the bulbs but not on them.

If the TPU gaiter makes a gap on the bulbs, then you may want to try to adjust the back strap in lower position.

If the back strap sits on the bulbs, then you may want to adjust it to a higher position.