The rivet holds the strap for the drop box and is fixed directly to the abdomen plate. The rivet goes straight through the plastic ammo section, elastic, webbing and then the abdomen in that order. Make sure you use a washer inside the abdomen plate to keep it secure.
Drop box elastic should be glued to the outside of the inner box.
You can do it any way you want but that is how the original ROTJ armour was