How are you going to run the line(s)? One single line that runs the whole diamater, or something snaking? Also, are you going to run it off of a 20 lbs tank (standard gas grill tank), or something bigger?

*IF* it were me, I'd find an old gas grill to donate some parts. Then use steel pipe with holes drilled in it to make a burner. The question is one single long burner, an "X" shape, or a snaking twisting burner? In any event, I'd feed the burner from more than one location to ensure an even flame along the burner.

Maybe something to consider would be to look at propane forges. Those guys are fabbing some great things and it may give you some ideas for burner construction.