Don't put a battery in the trunk unless you vent the trunk to the outside somehow. Since the trunk is sealed to the outside all the toxic gas from the battery charging will pass thorough the back seat and into the passenger compartment.
The f150 most likely had a factory dual battery option (usually for a Diesel) so you can puck up the second factory battery tray and bolt it in easily. Then isolate it with an isolator, which is available for under $50 from most automotive stores (mine came from NAPA, is a surepower with Napa's name on it).
The thing to remember about those jump start boxes is they have a battery too so it has a finate lifetime which will be about the same as the vehicles battery so after a few years your jumpstarter needs replaced too. plus they won't help with a really dead battery, they are only good if you left your lights on at the store, they won't help when its 10 below zero in a blizzard.