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Some people use magnets, but you'll be much better off with redundant attachment methods. For my master chief build I used a combination of velcro and sew-on snaps, both if which you can pick up pretty cheap. If you only have one method, i.e., magnets, you run the risk of it failing.
I also think, if I recall correctly, that using strong magnets aroind the body for extended periods of time can be very harmful as it can pull the iron and blood in your body around and even cause clots... I dont know if this is true...