Etrog Boxes Basic

What makes Etrog Boxes Kosher?
What makes an Etrog box Kosher, is the Etrog inside

Etrog Boxes are used for:
Used to protect the holy Etrog which is used for an immportant "Sukot" mitzvah

What is the tradition behind Etrog Boxes?
 "And you shall take for yourselves on the first day [of Sukkot], the fruit of the Hadar tree, date palm fronds, a branch of a braided tree, and willows of the brook, and you shall rejoice before the Lord your God for a seven day period." The fruit of the Hadar tree is Etrog