A gently rising mountain range up to 100K feet would do the trick nicely.
I wasn't part of the "old" conversations regarding this because when someone on the message board says it's been talked about before, that could mean 10-15 years ago!
My opinion on it is similar to Vraciu's approach (not sure if that's good or bad V!), but it requires some new maps. Now, not being a terrain editor like some of our dedicated community *oink*, I'm not going to speak TOOO far out of turn, BUT:
Isn't it possible to restrict the "off-map" flying by literally designing barriers at the terrain edge? Why not a large mountain range?
ORAuto-Puffy Ack on speed/meth; the further "off-map" you get, the more lethal the ack gets.

At map edge, get a little puffy reminder. Keep going off map edge = johnny puffy-ack gunner get's increasingly better....