Maybe it's possible to construct a small mobile wall of stone (5 ft square, but as a T-piece, so that it cannot fall or get pushed away) which lies within the size limits of the Shrink Item Spell (PH, 279).
If you manage to do so, you could use the spell to shrink the mobile wall. Consider making it permanent with the Permanency Spell so that you can shrink and unshrink the wall for an indefinite number of times.
Objects changed by a shrink item spell can be returned to normal
composition and size merely by tossing them onto any solid surface or
by a word of command from the original caster.
In order to speed that up and to let the wall appear at a convenient range you could use a quickened version of the 0-level Spell Launch Item (Spell Compendium, 130) making it a 4th level spell effectively. That would - as a swift action - give you a wall exactly where you need one.