How do I set an attr_accessor for a dynamic instance variable?

this answer doesn’t pollutes the class space, example.. if i do mine.my_number 4 then the other instances of Mine will not get the my_4 method.. this happens because we use the singleton class of the object instead of the class.

class Mine
  def my_number num
    singleton_class.class_eval { attr_accessor "my_#{num}" }
    send("my_#{num}=", num)
  end
end

a = Mine.new
b = Mine.new
a.my_number 10 #=> 10
a.my_10 #=> 10
b.my_10 #=> NoMethodError