I use a "range bag" similar to this one.
http://www.unclemikes.com/adtemplate.asp...bcatky2=2000201Mine is a cheaper off brand. I have a full size pick-up so I set it in the center and wrap the carry strap around the seat back to prevent it moving around in a sudden stop. With the top closed, it makes a nice arm rest.
Mine has a better lid. Two zipper type with a soft top that I just roll back out of the way.
Easy to remove when parked and also doubles as my EDC when I need to carry more but do not want to use a backpack.
Check your local "cop shop" or gun store. Very common item with lots of variations in lay-out and price.