cist : chest
The Illunse word for chest (box with a lid) is cist. Cist is an uncommon last name. Cist means chest in Welsh.
This word is a mixture of the Old English word for chest which is cist (or cyst) (chest, casket; coffin; rush basket), and the Latin word for chest which is cista (chest / box, usually made of wicker; box for sacred ceremonial objects; ballot box).
cista : chests
The Illunse word for chests (nominative plural) is cista. Cista is a unusual last name. Cista (or Čista) is the name of cities in Serbia and Croatia. Cista (or Čistá) is the name of cities in the Czech Republic. Cista means cyst in Serbo-Croatian.
Chests in Latin is cistae. Chests in Old English is cista.
My Illunse words for chest and chests are the same as Old English, and my word for chests is the Latin word for chest. I decided against mixing up the letters. The Old English word was derived from Latin, and the Latin word came from Greek.
I didn't find Quenya or Sindarin words for chest.