A word of caution when placing heavy (> 1 pound) items on the floor of your vehicle - especially if they fit under the seats. A 1 pound item becomes a deadly missile around 30 miles per hour and may thwart an attempt at an emergency stop if it impacts the accelerator pedal. Something stronger than velcro will be required to prevent this eventuality. I have experienced this with a text book and was almost killed during an attempted emergency stop during college. Since then I have banned all objects from the drivers leg area and in vehicles such as my van and jeep where the back seat room will roll under the front seat - I have banned floor items altogether unless the detritus of loose light obstacles or other blockades are in-place.

In my van I use one of those aftermarket console thingys to prevent the passenger side detritus from migrating into my leg space and I use large 1 Gallon spring water bottles behind the driver seat to prevent things from migrating from behind - BTW that gives me about 3 gallons of water - all set except when it freezes. I use the Poland spring screw top bottles they are very sturdy and I have some that are nearing 3 years old and still holding up. I refill from the tap, rotate the stock, and drink as needed. I have yet to have anything grow in my bottles un wanted. (I am lucky to have a decently clean well under my tap.)