Does anyone make a heel fit pad that uses a flow material like used in the Zipfit liner?
You can make your own "fit pads" that are custom-designed to fill the gaps in your boots. All you need is scissors and a box of wide (2" or more) sticky-backed velcro that you can buy at a fabric store or online. Be sure to get the wide Velcro, and be sure to get Velcro and not a wannabe competitor.
Cut the soft strip, the one that feels like cloth, to the shape you need and stick it onto the outside of your liner. That glue on the back is very strong and does not tend to slip around. You can even stick a second layer on top of the first for more thickness. And a third layer on top of that. If you find the pads you make are sliding out of position on your liner, reposition them and cover with duct tape (the good stuff). Your liners will acquire character.