Crafting and upgrading in Inmis

The first backpack is crafted from leather around a chest:

 [L]
[L][C][L]
 [L]

Each later tier is the previous backpack surrounded by that tier's material — iron for plated, gold for gilded, diamond for bejeweled, blaze rods for blazing, and so on. The recipe book lists the exact pattern for every tier once you have the ingredients.

Upgrading keeps your items. When a filled backpack is used in an upgrade recipe, everything inside is copied into the new one, so you never have to empty a backpack before improving it.