Just got a catalog in the mail form a surplus company that I may have used a while back and noticed some rare earth magnets at dirt cheap prices. http://www.mpja.com/
Put Rare Earth in the search box. I have not bought any of these but if you are doing a DIY masking or other project with magnets these would be a good deal
I'm considering using these magnets rather than finishing nails for securing the border to the frame. I'm pretty sure these will be strong enough - I was thinking 3-4 pairs on the top/bottom and 2-3 pairs on the sides. As far as fastening the magnets in the wood though, would it be better to glue the magnet on its underside and have the surface of the magnet flush with the surface of the wood or slightly recessed and then a layer of glue on top that would be flush with the wood. I'm concerned that I might not get a glue strong enough to keep the magnets in place.