I use a Spot2 as an owner service
on deliveries. In tracking mode using lithium batteries I get two or three days out of a set of batteries. The system isn't up to life-safety standards so be sure your friends and family
(or clients) understand that they shouldn't freak out if the signals stop for a while. That said it has been pretty reliable for me.
I use duct tape or blue painters tape to stick the Spot on the inside of a hatch
cover. That provides good view of the sky and keeps the unit safe and dry.