The box came from Lowe's like others have used. I used a heavy steel cable and attached it to the box with some U bolts. Then I cut and crimped the cable that it also serves to hold the box in place and keep it from sliding. With the plastic part of the soft cover in place, I can't open it fully in place.
While not completely secure for storing gold bars while parked in a bad neighborhhod, it is fine for storing some tools for occasional road trips.