Pattern for large hash creation?

I'm working on filling a large (155 fields) PDF using pdftk, and I find myself creating a massive Hash with the PDF's field names, and the values to be filled in.

For my specific implementation, I'm using Ruby, and have a PdfJob object which collects all of the data from a model object into a hash, and passes that hash into a wrapper around pdftk stored in $PDFTK.

class PdfJob
  def perform(model)
    $PDFTK.fill_form('path/to/fillable.pdf', Tempfile.new, Fields.call(model))
  end

  private

  class Fields
    def self.call(model)
      {
        OWNER_NAME: "#{model.first_name} #{model.last_name}",
        TOTAL_PRICE: calculate_total_price(model),
        FOO: 'bar',
        # 152 more lines of key/value pairs.
      }
    end

    def self.calculate_total_price(model)
      # Most of these methods are multi-line.
      model.item_relations.sum(&:price)
    end

    # about 50 more class methods to simplify assignment in #call()
  end
end

This is a heavily-simplified example, but I think it illustrates where I've found myself.

What I'm looking for is a design pattern or some other way to break up this hash into multiple classes, methods, or some other unit so I don't end up with a Fields class with nearly 250 lines of near-untestable key/value pairs and helper methods.