I don't know if this works across the board but it worked for me.
I simply right clicked in the default preview screen and checked 'Lock Preview'. When you do this, it doesn't allow you to interact with the sources in the current scene. You can lock/unlock/hide/unhide a source but you can't resize it or anything. So, this just seems to me that it also locks the dock in place as well. The preview screen dock is the main one that you cannot pick up and move around like you can the other docks.
So the key is to get your preview dock in a place where you want to keep it indefinitely, lock it down, then set up your peripheral docks.
Like I said, worked for me, but your mileage may vary. Good hunting.