I used something similar (bento box?) when I commuted on my diamond frame for the badge. Mine actually zippered up completely for a more secure feeling. On my recumbent I used to slide it behind my cell phone in the cell phone holder.

You could do the same for a toll tag I guess. Oh and uhm.. don't try and track stand or go really really slowly while you scan your badge or you may end up on a blooper reel and be known by all security folks like I am