Get a couple tie-downs and strap the spare to the floor.
If I were still into unpaved adventures, I'd consider carrying a spare on a roof rack. Murphy's Law, if I leave the spare at home you can bet I'll get a flat in the boonies and need it. Getting tie-downs to secure the spare on the floor is probably the best way to go. Was a couple months ago that I actually put air in the spare for the first time in 12 years. As you might guess, the spare tire was empty.