Thanks Giles, but it seems that I have a more complex scenario -
Even if I put the food (they're in those chinese take-out containers) in a bag within another bag, the inertia from braking/turning will make the food slide or even collapse within the footwell. I found out that food container placements play an major role to minimize this problem. First of all, putting the food in an open box with no space is the best way to go, although huge inertia will make the food 'sqeeze' together which ends in distortion. I haven't tried the seatbelt method yet, so I might try that with the box next week.
Also, how do you guys define 'carefully'? When I drive within time constraints I tend to use 'the line' whenever possible to limit the traction usage. On a 35mph turn I can go 60mph without causing the food to move. I stay at least 3-4 car lengths while driving. Is that careful enough?