How to find the right size controller back?

So I have lost the back to my xbox one controller. Problem is I don't know what size my controller is or what size the controller back is. Now if you're gonna tell me they are interchangeable don't answer because your wrong. I have the old gen xbox one not one s or one x. My friend gave me the back of his extra new gen console controller and it does not fit mine. I'm sick of my batteries constantly falling out please help.

:)