Jono- I'm still trying to get you an answer to your "5 x 5" question. Of course I now hear that in just about every movie I watch. :-)
A Berklee friend once told me that F# Phrygian was really creepy. I agreed. She also told me of a very scary F minor or F# minor chord. It was one of those "F#m 7+5 add 11" chords; I'm sure someone here will remember it. Keeping it in that key might raise some hairs on backs of necks. :-)
Check out some classic movie soundtracks too- Jaws, Psycho, Halloween 1 &2, The Fog, Lon Chaney movies, Bela Lugosi movies, and of course, Strange Brew.
As a general rule, keep it in a minor key.