Looks like the bus bars need a little push. You should be able to catch a few threads with a nut.
If that is flattened pipe, notch the lower one prior to flattening & do the same on the end of the upper tie piece. I'd then reverse your order of assembly placing the tie piece below the cell bus bar.
I might also sweat the pieces together by bolting & soldering prior to installation. Use a stainless bolt, nut, & washers to clamp them for sweating.
I've had copper become brittle from over-working it. Take care!Or just put the bar on a anvil, put a chunk of steel bar over it where the connection will go, and whack it a few times with a hand maul to thin it out. Annealed copper is soft and it squishes pretty easily.