Like some people suggest with wood slats and zip ties, I use aluminum angle. You can get it at big box hardware but if you have a metal supplier nearby it will be a lot cheaper. I don't remember what profile I used but whatever is smallest will work fine. It doesn't need a lot of strength. With wood prices like they are the aluminum is probably cheaper.
At a metal supply the angle will come in 20ft length. They will cut it to size if you ask them to, often with no charge. All you have to do is drill the holes and zip them in place.