What I think of... or at least what I would think of if I were picturing a haunt with the gates of hell, are just solid flags cut into triangles instead of rectangles. Something that can be done easily by getting sheets or whatever. Black and Red. LONG. Not a dinky flag, but something that if it were on an 8 foot pole would touch the ground or close to it.
Awesome if you can get a bit of wind, but still intimidating none the less... at least that's my opinion.
Best way to describe it is something you'd see on the top of an old ship, or something you'd see in medieval times.
Here's a photo of the length/style, NOT the color of course.
Some more reference photos. Medieval battle flags are really what I'm thinking... just solid instead.
I realize they may not have anything to do, literally, with satan/hell... just thought it'd be cool.