Flat-bar will serve the same function as magnets if you're exposing vertically. All you need are two lengths. Lie them down along the horizontal dimension. Less of a hassle, easier to set up from print to print. I've never had a problem with holding the paper flat using this method, whether RC or fiber. Perfectly sharp, corner to corner with wide open apertures.
As for aligning the paper to the baseboard... I use a sheet of 3/4" mdf cut to the width of my paper. Just line up one of the horizontal edges and you're good to go.