a thicker footbed might help, but I would also suggest trying tongue pads. these help secure the heel by adding volume to the boot/shoe above your instep. a fit oriented or large department shoe store would likely be able to give you some or sell you some for very cheap. a full service shoe repair will likely have a much longer tongue pad that is intended for boots. either way it may help, if you can only find tongue pads for shoes, get at least three for each foot so you can play with lining them up the tongue of the boot, or possibly layer them up 2 pads thick.
to try the tongue pad only use part of the sticky back at first to mess with positioning it.
hope that helps