Think of the most frightening thing you can imagine. As reported in the "Art of Star Wars: The Phantom Menace" book, George Lucas asked his production designers to come up with "the most frightening thing they could imagine" when designing Darth Maul. To truly do justice to the character, you have to come up with the stuff of nightmares.
Dumb that design down substantially. While the finished Darth Maul design was still frightening and cool looking, it is nothing compared with the original production designs for the character.
Ignore the fact that nothing truly scary is small and cast an actor who is 5 foot 7 inches to play Darth Maul. Ray Park was cast primarily for the fact that he could do his own stunts, not that he could be terrifying.
Dub his voice with someone else. A little known fact is that Ray Park did not actually provide the voice for Darth Maul---his lines were overdubbed by actor Peter Serafinawicz. Fans will know him as "Duane" from the UK series "Spaced."
Give him four lines. Despite being the one truly good idea in an entire movie, Darth Maul only has four lines in "The Phantom Menace." To accurately complete your Darth Maul, you have to make sure he stands silently in the background for most of his (little) screen time.