Here is a working example using the aws-sdk-v1 gem, which is required until Spaces supports v4 signatures.

class FoobarController < ApplicationController
  layout false

  def index
    AWS.config(
      access_key_id: 'ACCESS_KEY_ID',
      secret_access_key: 'SECRET_ACCESS_KEY',
      s3_endpoint: 'nyc3.digitaloceanspaces.com',
      region: 'ny3'
    )

    bucket = AWS::S3.new.buckets['BUCKET-NAME']
    @s3_direct_post = bucket.presigned_post(
      key: "uploads/#{SecureRandom.uuid}/${filename}",
      success_action_status: '201',
      acl: 'public-read',
      content_type: ''
    )
  end
end

<%= form_tag(@s3_direct_post.url.to_s, enctype: 'multipart/form-data', authenticity_token: false, enforce_utf8: false) do %>
    <% @s3_direct_post.fields.each do |field, value| %>
        <%= hidden_field_tag field, value %>
    <% end %>
    <%= file_field_tag :file %>
    <%= submit_tag 'Upload' %>
<% end %>