Vortex 27-60x85 Arca Swiss Adapter Plate That doesnt suck

Yeah, there's lots of them out there. Look at plates meant to fit on the lens collar foot of telephoto lenses, those can have a rear raised lip to prevent twisting. A search on ebay for "arca lens plate" will show you lots of options.
 
My guess is that this is your huckleberry.... http://www.reallyrightstuff.com/MPR-73-73mm-rail

I don't have a spotter to throw it on, but I do own one of these that I use on various cameras that don't have a specific plate made for them. It includes the MPR-C "bracket" which is basically a user configurable right angle stop that you can use to keep the rail from rotating. It's what RRS recommended to me as a general purpose adapter. Like all RRS stuff, it's super well made, and very solid.
 
The foot on this spotting scope is not an Arca Swiss foot and will not be compatible without attaching an Arca plate. The foot on this spotter is the Manfrotto RC2.