I think you just have to practice sounding like the film troopers.
I troop as ANH Darth Vader and use a Vortex II voice changer with two AKER amps. I still have to deepen my own voice, slow down my speech and over enunciate to sound like James Earl Jones. It takes time to get it right...
I annoyed my wife and kids after I got my Vortex because I walked around the house for two weeks reciting lines from the movie all the time.... it was fun! Lol!
I hope this helps a little!