In order to keep things simple, I thought a simple hex grid would serve as a decent rear cover for BX3e (like the one on the rear cover of the Gamma World rules from the same time period):
Maybe a map of the Grand Duchy of Karameikos would be a better option. Or…
Maybe the Expert rules cover on the rear cover. (?)
I think, looking at the three of them, that I’m partial to the map of Karameikos. Hmmm… What are your thoughts?