I have used Thunderrift as the Outer Plane of some as yet unidentified Immortal.
And the Players are actually dead souls who have traversed through the Gate of Eternal Rest on Limbo after their Mortal demise, only discovering there is no eternal rest, only more adventuring....as the Immortal desires to have some angels,,,and souls in the eternal Rest Plane (actually mostly the Home Plane of that Immortal) can continue adventuring and become Angels instead Immortals (actually they are then a sort of Immortal creation/alteration)...at least that was suggested in Bruce Heard article on Death/Limbo and beyond in Dragon Magazine...
And I had at roughly the same time laid my hands on the Thunderrift material , recognized the limited area, and used it as described above.
As such the Mountains maybe passable, yet are somewhat different in appearance;
1;...a vast Plane with dotted valleys all over like Thunderrift, each a bit different, This may be a Sphere, Spheroid, or Horizontal Plane, mostly Limited in size
2;...A infinit limited Plane Like Limbo, where one side exits on the other
3;...A limited Plane, where the edges of the Plane end into the Astral Plane, or just end into a forcefield
4; something completely different; another Plane next to the one the PC's are in
Eitherway the "impassable Mountains" would be the Planar border to its edge, or the walls of the valley as created by the immortal. sort of a Dish Plane with a dome air-envelop within the mountains grasp
The next Image I used as a combination of 1 and 3 how I did it...the various valleys can be seen, as the edge of the Plane and the Astral(or Other) Plane beyond, seen by the adventurer breaking through it...He might now become an angel of the Immortal in question
This system clearly keeps the Thunderrift restrictions, yet also enables for PC's (following the Thunderrift DM screen adventure) to return to Mystara....when passing thus actually reincarnating into a new body mimicking there own... to be alive anew...yet unknown to them