$ git clone http://github.com/indirect/middleman-heroku-app.git mysite
$ cd mysite
$ heroku create --stack cedar --buildpack http://github.com/indirect/heroku-buildpack-middleman.git
$ git push heroku master
The only expectation is that middleman build will generate your site into ./build. That's where Rack::TryStatic will look.
You can customize the 404 page that's served if TryStatic can't find a file by editing source/404.html.erb.