Snowboarding boot inserts, goes between the outer shell and the lining. They're designed to increase heel hold, by filling in the space around the heel. Sized for a US 8.5 boot, it can be scaled in z to adjust the thickness or X and Y to adjust the sizing.
The triangular pattern on the bottom was generated with infill, and added to increase the friction on the bottom surface. It should be printed with a flexible filament (unless you want to heat mold it).
-- 3D Supply Source Flexible Red