I plan on doing that too, but haven't started experimenting. I am going to try rubberbands or small springs, magnets & a trigger latch that holds the blades closed under tension. I have a hollow grapple rocket, so that will make it easier.
You could just make a cardboard mockup of the back side of the jet pack, then put it against the armor and try different measurements until it looks right. Then use the cardboard as a template for the final cuts.