OK, just got back from the test ride and can happily report that the god of risers has smiled upon me. No handlebar roll. So, here's what didn't work, what might have worked, and what probably worked. Shims - didn't work. There was insufficient room in the clamp to accommodate a shim of any thickness. Roughing up the inside of the clamp around the handlebar - might have worked, probably didn't hurt. What probably worked was reinstalling the riser while paying more attention to the process. I thought I had originally installed them in keeping with the instructions and as Randy recommended above, but I probably didn't. This time I made sure I paid attention and the result was success. So, once again, my mechanical ineptitude teaches me humility.
I also learned that the folks at Barons are first-rate - working hard to make sure that their customers end up happy with their products. And the 'Liner Risers are a great product. Thanks to them and to Randy for gettin' me straight.