You can use a faceplate, but it limits what you can do with the back side of the call in my (brief) experience. You might be able to fashion some way to mount the call back to the faceplate with a bolt to work on the back side.
I recently purchased a 4 jaw chuck from PSI (
http://www.pennstateind.com/store/CUG3418CCX.html?prodpage=1CU). It makes turning pots SOO much easier. The pot can be mounted to the jaw in several ways, from both sides, making it a lot easier to work on each side.
Prior to getting the chuck, I was gluing a piece of scrap on to my blanks, mounting it on a faceplate, then taking hours to sand down the back side after cutting it loose. I fashioned a homemade mandrel of sorts to slide the pot on to in order to sand the back, but it didn't work nearly as well as the chuck does.
ETA:
They say a picture is worth a thousand words... so here are two thousand...
This is the 'mandrel' I made. As you can see, I had sandpaper glued to it to grip the call. I was only able to barely sand the bottom of the call enough to make it look decent. There are, I'm sure, a million other ways to do this... I just made it happen with what I had available to me at the time.
And here's with the chuck, allowing me to cut in on the back side of the call, do some really heavy duty sanding, and not have to put that center hole in (although, I get a better sound with a center hole on slate, better without center hole on crystal... go figure).