Seem like another instance of impossible folding devices (see mass effect for other examples). Still, you could do somethings that would get you a "folding" shield. However, the unfolded version would most likely not look smooth or hold up to any kind of pressure. The first idea that came to mind was a telescoping type system; however, this means that the unfolded shield would have clear ridges where each layer overlapped. There is also the issue of constructing such a design, which would require more percision than may be easily possible (not sure on budget or your resources).
I managed to make it work using small hinges. This was done as a commission for a friend of mine.
Edit: Forgot to mention, on the back of the shield is a set of hooks for a length of small diameter pipe to clamp into and hold the shield open when folded outwards, along with magnets on the inside edges to assist in holding it open, as well as folded.