I use a Preston innovations 50" flatback with fibreglass arms. It's a good brolly ,but outside your budget. My other brolly is a 50" Daiwa mission, another good lightweight brolly, I pole pole fish as well as use rods, I only it as a flat back if raining hard behind me and float fishing,
As for zips in back panels, I have seen a few give up the ghost,