Maybe a little more description or a picture. I'm going to guess a Halo 3 Mk 6, (Halo 4 is also a Mk 6 and very different). Also, I'm going to guess that the part needing expanding is the larger end. My recommendation would be to guy buy a yard of vinyl and some thin flexible plastic (even reused like from sour cream containers or something). Now, the slats along the inner thighs are actually meant to be a flexible material. Cut out the plastic to the shape of each slat and then wrap in vinyl. Again though I'm more or less guessing and it's not a quick fix but a pretty snazy one imo. If your problem is that you can't get it past your foot or your knee then yes you would want to split it and there are many options including, buckles, velcro and magnets. Good luck!
