I like the look of that box. It fits nicely with the bike’s aerodynamics. I would bet the two holes you pointed out are specifically there for use in fastening around the front of the seat tube. I would try something like a rubber band, or a small piece of inner tube cute thin enough to fit through the hole(s) (make the holes bigger if necessary). Then, keep the band from going back through the hole with a small piece of dowel rod or wire. — Dan