slug.rb

module Slug
  def self.customize(field: :name)
    Module.new do
      define_method :to_param do
        public_send(field).downcase.gsub /\W+/, '-'
      end       
    end
  end
end

class Cat
  include Slug.customize(field: 'hello')
end

# you can go further if you want...

module Slug
  def self.customize(field: :name)
    Module.new do
      include Slug
      
      define_method :to_param do
        public_send(field).downcase.gsub /\W+/, '-'
      end       
    end
  end
  
  def non_customized_method
    "everyone gets this"
  end
end