The modifications suggested do reduce the contact between the arrow and arrow shelf, but I don't think the arrow shelf is what's causing the fishtailing. There are a lot of older bows around from the 50s and 60s that have flat shelves, and most of those shoot very well if the arrows are properly matched to them. Maybe it's something in your form, arrow spine, even brace height, etc.
Just so as not to overlook the obvious, you are using feather fletched arrows, right. Vanes don't fly well off a shelf, regardless of how it's constructed.