Magnets are probably the best way to go without having the attatchment mechanism viable. Neodymium magnets are very strong but usually too small to use..
In this build that is VERY old, jacket brackets are used. They seem like a reasonable idea, but I have yet to try them. I may use them soon but if you get to it before me, let us know how it goes.
Halo 4 SRS99-S5 Sniper Rifle - EVA Foam Build [COMPLETE]
He also has a link at the end of his thread that takes you to another of his that explains the jacket bracket and how he used it...
I would try and install the magnets inside of your props and armors before completing so it isnt visable, but whatever works!