Not often, but I do occasionally pick up a burger and fries for lunch or dinner from any of the assorted establishments in my town. I always set the sack of food on the passenger seat, and usually remember to hold it there when I'm braking or turning. Last trip I was distracted with snowy street conditions and forgot to hold the sack when I braked at a corner.
Thanks to Newton's first law the fries found a new home on the passenger side floor.
Vowing never to have to worry about this again I've come up with two potential solutions. The first is a simple cardboard box to place the food sack into. Fast food sacks are usually 5" x 7" or 3.5" x 5". This box is sized for the larger sack, but has a removable divider to partition it for smaller sacks. It also has a flap that tucks between seat and back which keeps the box in place and contents upright.
Another bug is that sacks often open up. On cold days they will lose heat quicker if open. This solution keeps the sack closed and hangs on for cornering and braking. A 2" strap and a chip clip. Held between seat back and head rest with a 1" dowel the strap wraps around. I can adjust the strap length if needed.
The best solution may be a combination of both ideas. A smaller chip clip could be left in the box to use with each acquired meal. Pinch the top of the sack closed to retain any heat while also keeping the sack from tipping over or sliding off the seat.
4D