barkbusters makes a shorter insert that solves the too long problem but doesn't match the handlebar size. They come in 14 and 18mm. The bars are 16.6mm. Per a suggestion above, I wrapped some rubber sheet around the 14. Clever idea, but did not work. The end of the 14 is the part that expands, so with only a narrow expanding ring next to the expander bold, not enough area to keep the barkbusters from drooping.
Something less compressible than rubber sheeting perhaps?
Energica sells expensive bar ends as an option for the SS9+. I don't know what the internals look like. Perhaps that might provide a solution?