Using a heat gun or a hair dryer, you can actually form the calf pieces to fit together, closing like a clam shell, with no need for either velcro or glue in the back. I did this on my FX. I just glued the front together, using the ABS cement that came with the kit, giving it at least 48 hours to set. Then, using a regular hair dryer, I heated the calf pieces to the point where I almost couldn´t hold them, feeling the plastic softening somewhat. After that, I just bent them, using my hands, until the pieces were shaped so they would naturally close in the back.
Doing this takes a while; you have to heat the pieces, then bend them, then heat them up again, bend them some more, etc. - but the final result is well worth it, I think. Apart from the screen accurate look, you´ll get a pair of calf pieces that will flex in the back, adjusting to the muscles flexing when you walk, or climb a set of stairs.
It´s hard to explain exactly how you should bend the pieces, but I think you´ll discover it for yourself, once you have the pieces in hand. You gotta sort of bend the one piece with the "upper" lip inwards, hard, and then bend the other piece, with the "lower" lip, to go under under it. When done correctly, the tension between the two pieces should be enough to keep them closed.
To really make sure the calf pieces stay closed, I recommend putting a strip of velcro, not in the joint itself, but on the inside of the pieces, across the joint, somewhere near the top of the calf.