final Location l1 = player.getLocation().clone().add(10, 0, 10);
final Location l2 = player.getLocation().clone().add(-10, 0, -10);

final double percentage = 70;

for ( int x = Math.min( l1.getBlockX(), l2.getBlockX() );
        x < Math.max( l1.getBlockX(), l2.getBlockX() ); x++ )
{
    for ( int z = Math.min(l1.getBlockZ(), l2.getBlockZ());
            z < Math.max( l1.getBlockZ(), l2.getBlockZ() ); z++ )
    {
        Material material;

        if ( percentage > (Math.random() * 100) )
        {
            material = Material.GLASS;
        }
        else
        {
            material = Material.AIR;
        }

        l1.getWorld().getBlockAt( x, l1.getBlockY(), z ).setType( material );
    }
}