If you want a really straight strongback, use a steel wall stud. There are numerous sizes, what you want is one that approximates the size of a 2 x 6. You'll have to go to a building supply specialty store to find one, but it's worth the hunt. They aren't expensive and the guy at the store will be intrigued by what you're going to do with it.

They're straight as an arrow, galvanized and can be cut with a pair of tin snips. Sheet rock screws bite into them like a wild dog.

Carl