I'd go west from Tower Junction, head for Billings on 212. See the little Big Horn Battlefield for a few hours and head for Devils Monument the same day on 212. If you are talking about the Cody route and thru the Big Horn Mts its only 52 miles north on I90 to the Custer battlefield and from there 212 will take you to Devils Tower. Nice KOA right at the base. From the Tower you are right at I90 for the trip home. Four hours is enough at the battlefield and Devils Tower is a matter of see it, take a few pics and be on your way early the next morning. Its doable in one day. Sun set is pretty late that time of year.
