I was borrowing a set of backcountry skis for my wife from a friend and her boots are a size shorter. We needed 5.5mm gap at the heel and had to use an Allen key to get the space right. Finding these Dynafit heel spacers is really hard so decided to create one I could print. Make sure you print the right one for your binding model, this article has a lot of them: https://skimo.co/tech-binding-heel-gaps