Use a sub-subdomain on Heroku review apps with DNSimple. Run this task from your app.json in your postdeploy script.

create_review_app_subdomain.rake

namespace :staging do
  desc 'create subdomain DNS record for Heroku review app'
  task :publish_dns do
    require 'dnsimple'
    require 'platform-api'

    STAGING_DOMAIN      = 'mystagingdomain.com'.freeze
    DNSIMPLE_ACCOUNT_ID = .freeze
    heroku_app_name    = ENV['HEROKU_APP_NAME']
    subdomain          = heroku_app_name.match(/.*(pr-\d+)/).captures.first
    wildcard_subdomain = "*.#{subdomain}"
    heroku_domain      = "#{heroku_app_name}.herokuapp.com"
    heroku_token       = ENV['HEROKU_PLATFORM_TOKEN']
    type               = { type: 'CNAME' }
    standard_sub_opts  = type.merge({ name: subdomain, content: heroku_domain })
    wildcard_sub_opts  = type.merge({ name: wildcard_subdomain, content: heroku_domain })

    client = Dnsimple::Client.new access_token: ENV['YOUR_ACCESS_TOKEN']
    heroku = PlatformAPI.connect_oauth heroku_token

    # Create records on DNSimple
    [standard_sub_opts, wildcard_sub_opts].each do |opts|
      client.zones.create_record DNSIMPLE_ACCOUNT_ID, STAGING_DOMAIN, opts
    end

    # Let Heroku know about records
    [subdomain, wildcard_subdomain].each do |subdomain_type|
      hostname = [subdomain_type, STAGING_DOMAIN].join('.')
      heroku.domain.create(heroku_app_name, hostname: hostname)
    end
  end
end