Help with MeshPart/Back Accessory

Over the past few weeks, I have been having issues positioning how a back accessory sits on the player. When the accessory is played into the Character, it changes size. I am on r15, with the Handle of the back accessory being a MeshPart.

Goal

How It Appears
https://gyazo.com/de94a4becab1e8abcfe6cb5ad9ba89d6

Any help is much appreciated! Thank you for reading.