Or simply kevlar coiled around wire that gets bracketed to the wall?
Though... one thing you WILL have to remember is that when you light it the fire will come up, so letters will not quite look like letters unless they are rather large, the flame will obscure fine detail if thats what you're intending.
Unless of course you can find some way around that... limiting surface area would limit flames in certain areas.
In light of that, UCOFs idea sounds like it could work in that way, different sized ropes or some such thing?
I'm sorry, I really don't have an answer for you... but these are at least some points to mull over, or perhaps some lovely person in here will get a brainwave off this.
Good luck and most importantly.. take photos for us when you're done.
Perhaps you could tell us more about the setting??? Does it need to be able to be set up in different locations or is it able to be somewhat fixed? What is its specific purpouse other than its ultimate purpouse of looking awesome.
