what's the simplest way to keep a spare key fob in my new 2015 T&C touring L minivan.

Get a hide a key box that has a magnet on it. Stick it under the car somewhere convenient. Tape a new battery still in the package to it if there is room in the hide a key box.

http://www.thehomesecuritysuperstore.com/auto-safety-jumbo-xl- magnetic-key-hider-912-p=2281

Your front door lock can be opened with a key with out a fob. Your fob has a hidden key in it. You can pull out that key and have a spare cut from it and hide only the metal key to get you in the car. You can take the battery out of the Fob and store both in your console. Our ACE hardware shells the fob and key for $20 they can cut the key and program the fob. I had two extra done for my van.