If you have raised the outside edge of the matrix a fair amount, then the texture used will be stretched - there's nothing you can do about it - other than use a single coloured texture.
The second alternative is to create plains, texture them and align them with the edge of the matrix. This isn't difficult as you know the X and Z is 0 along 2 of the matrix sides, and whatever size you chose when creating the matrix for the other two.
The last option is to create the walls in a 3D modelling program, load them in and place them in the same way you would with the Plains option.
TDK_Man