The toe strap came off my daughters bike when she decided she didn't like them. The intent was to strap to the saddle rails but have wound up carrying it in my jersey pocket. I put my cell, ID/bank card and key in the wrap when I ride and have worried about it working loose and falling without me realizing. I am probably going to sew the strap to the wrap so that I can carry it on the seat rails.
Another option that perhaps would make for an easier installation under the seat would be to sew the ends of a 1" strip of leather on the outside of the roll. Then just run the toe strap through that when you fix it to the rails.