"But not keen on the idea of creating permanent or semi-permanent stuff in the proper outdoors"
Agree about not permanent (EH at Tintagel for instance) but I loved this at the Rollrights recently https://www.google.co.uk/search?q=witches+rollright&source=lnms&tbm=isch&sa=X&ved=0ahUKEwiO_IeY__7dAhXsLsAKHXu7AksQ_AUIDigB&biw=911&bih=409#imgrc=6T8dXT-3iXJDpM: