I've generally found with the small sandwich size ziplocks (get the cheap dollar store kind - not the name brand) that if you fill it till its full, gently seal it, and place it inside virtually any amount of pressure or disturbance will cause the seal to open. Hammer the sign into the ground deep and they're sure to loosen it just while trying to get the sign out. Then when it makes it to the back seat the contents spill everywhere.
That should work alright. But to avoid premature leaking and discovery could be tricky sometimes.
I'd think.