At the BCME 6 conference in Britain last month I caught up with John Hibbs (retired - but still very active - HMI) who discovered one of the original Size 3 Sphinx solutions. He had rediscovered the drawing of his solution, so, as an item of history a scan of it has been added to the record of his solution at:The fascination of the Sphinx seems to be in the way it replicates itself - 4 Sphinxes make a Sphinx is the starting point. Are there other shapes which behave this way? Certainly.
http://www.blackdouglas.com.au/project/sphinx/sfnx3sol.htm
John has donated some work from an old notebook which might start you off on the hunt. Since this work can be seen as a What happens if...? question developing from the Sphinx problem, it has been recorded on Page 4 of the Sphinx Album.