bottomplate

noun

Plural:   bottomplates

A layer, plate or board forming or reinforcing a bottom of a container, vessel, lake, etc·.

Examples:

Coating-based pans observably wear out with usage.  Sooner or later, a lousy aluminium pan eventuates with a typical wavy bottomplate (there is rarely dependable stainless steel or cast iron underneath the overpriced lamination).  (Source: J∙ Okram - Bartonian Potato Pancakes  ●  Cooking Recipe.)