Magnets or just attach it back there..
I dunno'.. it might be tricky to trust a magnet, I'd rather use a post or something and make a positive lock with it. Otherwise you run the risk of it falling, or someone grabbing it off your back and hauling ass with it.
However, if you decide to use a magnet, use 4 of them. Two positions on the rifle itself, and two positions on the back, to stop it from rotating.
http://www.magnetsource.com/Solutions_Page...chmaterial.html
Maybe that'll help with choosing a magnet.
If the magnet is too strong, it'll stop you from pulling off the rifle without risking damage. If it's not strong enough, and the backplate or rifle isn't adjusted to stop from slipping, it'll slip down your back enough to disengage the magnets if you jog it at all.