why dont you just do some coding to raise the outer edge of the world straight up?
xdiv = 20
zdiv = 20
wallheight = 50
make matrix 1 , 20000 , 20000 , xdiv , zdiv
for x = 1 to xdiv
set matrix height 1 , x , 0 , wallheight
set matrix height 1 , x , 19 , wallheight
next x
for y = 1 to ydiv
set matrix height 1 , 0 , y , wallheight
set matrix height 1 , 19 , y , wallheight
next y
This code will make a wall around the outside of the matrix. However, the wall will be slanted and without some other coding the walls will be the same color as the matrix. This is a good thing however because you can make mountains. That looks a little more natural then random walls around the place.
Formerly known as SFLM.