I went with the PacSafe 5 liter security bag that @Wendell posted and used it for a recent trip that covered 3200 miles across 10 states. There are many things to like about the bag but I'm not sure if I'll use it day to day for one holstered pistol.
For the trip I only had to use it a couple of times. Once to store my carry gear for a short visit to an NPE (church) and once to store carry gear and backups while in an NPE (college campus) for almost 2 days.
Here's a list of pros / cons as I experienced from my trip.
Pros
- Enough room for 2 holstered (JMCK IWB3 and pocket) and one cased pistol and 3 extra mags.
- When empty, flattens for easy storage.
- Can easily fit in a backpack when full.
- Seems to be sturdier than typical gun store lock box.
- Can be used for a variety of valuables.
Cons
- Due to how the cable and lock work, it's more difficult to be discrete in vehicle.
- Items are loose in the bag. Care must be taken when placing and retrieving firearms and handling the bag.
- More bulky than smaller lock box when storing under a seat.
- Lock that comes with bag is inadequate.
- With better lock cable is weakest link.
- Max shackle diameter of 1/4".