A curious thing, are you making a scrub blade for your #5? Regardless, like Mike said, you can do it freehand if you're handy at such things- I do the routine sharpening/honing of my curved blades and chisels freehand. You may be doig some final shaping of the edge ini your case, and if you're wanting something more exacting, and easy way is to use the blade (in the plane) to cut that shape in a hardwood board. Then use the "scary sharp" method with the sandpaper layed in the curved shape in the board. I've done that before for scrub plane blades, gouge chisels, lathe chisels, carving chisels, etc, works for some other odd convex shaped cutting edges also. For some concave shapes, simply wrapping the sandpaper around the appropriate diameter dowel. I use the dowel method a lot for bench and lathe gouges.