way back in time, when I bought my C14, I did a review of a pilots backrest (which was a horrible p.o.c.) and took a picture you might find useful....
it seams that the seat actually may have been planned to have a "passenger grab strap" in place, like the old bikes of the 70's era. There are notches in the bottom edge of the pan, and also holes in the pan sub structure, that I'm sure were for that purpose... probably when they did it though, it looked hokey to the design artists, and they ditched the idea....
take a look, and think about adding a strap for the passenger's usage...