I deal with this situation. One of the finest human beings I know, now in his 50's, was caught at age 19 with 22lbs of pot
.
Part of my answer is to carry in a fully enclosed fanny pack on my person at all times, not (for example) dropped onto the floor of a car or the car's trunk.
The basic idea is, you don't want them ever to be accused of being in possession of the thing.