I have a Dava flatpick with a section in its middle that is both thinner and ridged. The idea was that if if you grip it down by the tip it's a thick heavy pick, but if you grip it back behind the thin part, it's flexible like a thin pick. I was thinking that if you need a flatpick, this one might be less slippery than the ones you mentioned trying.
http://elderly.com/accessories/items/PK80P.htm
http://elderly.com/images/accessories/PK/PK80P_close.jpg