Around here they make most of them with pipe and supporting braces but also use other stuff. The end fence supports can be perpendicular or go out at 45 degrees to the fence height. Main idea is to have enough spacing (about 4" or so) between the pipes so cattle are afraid to step on it. I could probably drive 10 miles and find 20 or thirty of them. Go drive the back roads around some pastures and you will see some to model.
Sorry, just reread your post and maybe you want a gate as opposed to a gateless cattle guard. Go to your nearest farm supply store. Pipe and tubing no smaller than 1" should work. Unless you like to weld and have metal laying around, they are almost cheaper to buy premade.