i just built me a cheap one today for a test....used 7/16 dowel rods....might go to 1/2 inch or larger on the rods but the 7/16 rod will work for now. just drilled holes 7/16 in the 2x4's and shoved the rods in. secured the 2x4's into the drywall with screws and the anchor in 2 places on each 2x4.

pic attached are the anchors that go into the drywall (short screws that came with the anchors weren't long enough) so I bought #8 2 1/2 inch screws

the metal brackets are much more expensive