In my original post (I started this thread) I suggested some aluminum washers (actually, they are aluminum gaskets for Honda oil pan plugs). They have a inside diameter of 12mm, so a little larger than the top pad. The thickness seems pretty close to 2mm. I then put some Gardner Bender Ox-guard under and on top of the washer, then put my bus bars (with holes for M6 studs) on the stud. Tightening down the nut seems to do what @Hedges
suggests in getting a good connection with the top pad, but also compresses the washer against the next level down (the weld shoulder). I haven't run any real high current tests, but I did run for 6 hours at 25A draw and there was no warming of the terminal threads or bus bars. My highest draw will be about 55A and even then for only 2.5 minutes at a time, so I think this will work well.