thank you guys I knew I'd find precious help in this forum. Will try to put this into practice before the next session. The fin saver looks a feasible option.
Thanks for the links Floyd, there's no piece of front section left, the whole front section just broke. so it's either the fin saver Ola H suggested or a device made of my own following Roger's instructions which proved very effective in previous occasions. meanwhile I'll have a look at any U.S. box fins available on the web in that size. all the best