Check out this new minimalist magnetic back-up mag holder from NeoMag. The pocket clip is made out of titanium with a powder coated machined aluminum and steel bracket. Your back-up magazine is then held onto the bracket with a neodymium magnet. They might not work with all magazines however, so be sure a magnet sticks to your mags before considering them. Currently they’re available for single or double stack magazines in .45, .380, 9mm and .40. They currently retail for $44.99 at Theneomag.com.