Multiple adjectives to further describe the bottom of something

[Tycholiz]

Im trying to describe the back cushion of a couch, el parte posterior de la sofá. The particular sentence is "there should be a unique corner back cushion to better fit the design"
Debe haber un cojín de la parte posterior esquina único para caber el diseño mejor

Muchas gracias por tus respuestos!!

[AngelicaDeAlquezar]

Im trying to describe the back cushion of a couch, el parte (wrong gender agreement) posterior de la sofá (wrong gender agreement). The particular sentence is "there should be a unique corner back cushion to better fit the design"

Debe haber un cojín de la parte posterior esquina (although there may be a specific expression for this, check your wording; it doesn't make sense like that. You need something to link "parte posterior" and "esquina") único para caber (Check your dictionaries. Caber means to fit when something can be contained inside another object) el diseño mejor

*** (Never use ending question of exclamation marks without their opening ones) Muchas gracias por tus respuestos (check your dictionary)!!

Back cushion could be:
- cojín del espaldar
- cojín del respaldo
- cojín de la espalda

As for a corner back cushion, I can only think of a very long expression like "cojín para el respaldo de la esquina".