Proof of Concept: create multiple, identical, AR models from a single form.

new.html.erb

<div>
  <h1>Add a thing to your box</h1>

  <%= form_for @thing, html: { class: "form-stacked" } do |f| %>
    <label>Name: <%= f.collection_select :widget_id, @widgets, :id, :name,
                                            {include_blank: true}, autofocus: true %></label>
    <label>Blah: <%= f.text_field :blah %></label>
    <label>Other stuff: <%= f.text_field :other %></label>

    <label>Count: <%= text_field_tag :count, @thing_count %></label>

    <%= f.submit "Add them!" %>
  <% end %>
</div>

things_controller.rb

class ThingController < ApplicationController
  def new
    @thing = Thing.new
    @thing_count = 1
    @widgets = Widget.all
  end

  def create
    @thing_count = params[:count].to_i
    widget = Widget.find_by_id(params[:thing][:widget_id])

    new_things = (0...@thing_count).collect do |i|
      Thing.create(params[:thing]) { |t| t.widget = widget }
    end

    if new_things.all?(&:valid?)
      redirect_to(things_path, notice: "Things with #{widget.name} have been added to the box!")
    elsif
      @thing = new_things.find(&:invalid?)
      @widgets = Widget.all
      flash.now[:alert] = "Oops! #{@thing.errors.full_messages.join(", ")}"

      new_things.map { |t| t.delete }
      render :new
    end
  end
end

What about the following API?

# things_controller.rb
def create
  @thing_count = params[:count].to_i
  adds_things = AddsThingsToBox.new
  adds_things.add(@thing_count, params[:thing])

  if adds_things.success?
    redirect_to(things_path, notice: "Things with #{adds_things.widget.name} have been added to the box!")
  else
    @thing = adds_things.invalid_thing
    @widgets = Widget.all
    flash.now[:alert] = "Oops! #{@thing.errors.full_messages.join(", ")}"

    render :new
  end
end

Where most of the gunk of knowing how many Things to create, how to build them, and how to roll them back, is pushed off into the AddsThingsToBox object.