Flat liner?
I believe the answer is:
insole
(Other definitions for insole that I've seen before include "Boots may be holding it" , "Inner part of shoe" , "Padding in a shoe" , "Part of a shoe or boot" , "found in boot" .)
I believe the answer is:
insole
(Other definitions for insole that I've seen before include "Boots may be holding it" , "Inner part of shoe" , "Padding in a shoe" , "Part of a shoe or boot" , "found in boot" .)