what holds the remote mirror control in place?

what style door do you have? i have one car that the mirror control goes through the sheetmetal part of the door and its just a sheetmetal nut that screwed on, just like the chrome retaining trim piece. my other car im going to ad the remote ralley mirrors but it goes through the door panel and i dont have that piece, so it looks like the pic above is the piece i need too. looks easy enough to make.