Pixelized epsilon output file

I'm trying to get a 2D grid where I can set/reset each pixel to
air/dielectric values. I tried using a fixed input-epsilon-file but I
need to use an isotropic material, which is not supported by such
option. I tried by making an object list of blocks corresponding to
the "on" pixels, but I can't get them to be the size of a single
pixel. Is there a way to do so, or to access each pixel in the grid?