Car stereo installers have gotten quite creative at building false floors or dividers. If you have a false seat back or false floor (in the trunk) covered with the appropriate automotive carpet almost no one is going to notice unless they are looking for it.

You can get a fair amount of stuff in a 4-6" false floor or a 12-15" false seat back.

A cheap, large, nondescript duffel bag without any exciting logos will conceal a lot of stuff too. If you have kids in sports like I do we always have several extra duffel bags laying around collecting dust.
"There is not a man of us who does not at times need a helping hand to be stretched out to him, and then shame upon him who will not stretch out the helping hand to his brother." -Theodore Roosevelt