I ended up folding the copper over on itself to double up the thickness. This seemed to me to be the best approach to ensure consistent contact. I'm using 12 gauge speaker wires but some of my runs use 16 gauge wire. There is no way 16 gauge wire would stay connected as the plates cannot be tightened enough to hold it.

So I think doubling up the wire is the key!

No I need to get a replacement bracket since I stripped the hex screw on the first one when I over-torqued it to tighten it!