Just make up some L-brackets from that shit they hang garage doors with. Few screws in the box and the floor and youre set. Just be careful not to screw into the top of the gas tank. If youre still using that box thats was in the accord, thats how I held it in place in my old sentra. Worked perfectly even when driving like an asshole.
If its that small and light of a box, the velcro is your best bet.
mine has enough weight to keep it in place