The first thing I'd look at for causing the zippers would be the delay time. My guess is it's either too short or too long. Do you remember what the delay was on the F24? I'm absolutely certain it didn't go to 25,000 feet. 2,500 feet would be doubtful on an F24, especially with the extra weight of the longer body tube. Do you have a total weight for the rocket? What length and material is your shock cord?