Originally Posted by superslomo
There's a lid on the bento box that velcros over... I don't know if you meant adding additional velcro on top of that?
Yes. For a $100+ battery I'm not willing to trust the velcro on the bento box alone as you can NOT close it entirely with the battery in there diagonally. I can just see that thing flying out after I hit one of the famous Boston potholes at half speed even.
Also the OP has cables running on the under side of his top tube. I have my cables running on the top of the top tube so I can actually velcro the battery to the top tube and still have my rear derailleur and brake cables slide through the velcro. However the rubber contact surface for the battery will likely create an issue with the bottom sided cables.