i made this out of a draw slide (http://www.amazon.co.uk/DRAWER-RUNNER-MFI-EXTENDS-452MM/dp/B002A0QV42/ref=sr_1_6?ie=UTF8&qid=1339952748&sr=8-6) and various other bits. the straps are made of Velcro and it is stuck together with aruldite glue. the blade its self is a mettle cabab stick however i could not find a good way of connecting the spike to the body so i used blue tack witch works well enough. I used an elastic band so that the spike fires out however it dose not lock in place so it quite easily slides back in.
here is a video of it working: http://www.youtube.com/watch?v=zRK3Prk5-7g&feature=plcp