Though I haven't purchased one yet (it is kind of expensive) this looks like a good bet for easy field sharpening with a 5.25" X 2.5" case, 4" long rods, a pair of ceramic rods, a pair of diamond rods, and a 30 degree angle (the same as the Rittergrips):

http://www.agrussell.com/accessories/sharpening/a_g_russell_field_sharpener.html