I have found this great wee paper model pelvis which could be a wonderful resource for midwifery students. I am posting the link here http://weishinlai.tripod.com/pelvis/
There is also a good wee model (and a bit simpler) in the Hesperian Book for Midwives on page 448 and 449. The chapter on homemade tools will download if you click this link http://hesperian.org/wp-content/uploads/pdf/en_midw_2013/en_midw_2013_25.pdf . It is a book written for midwifery students in developing countries who may not have limited access to resources.
This is another resource for students to build clay model pelvis. It may be useful but seems to require some precast pieces. http://www.iuga2013.com/handouts/docs/handout24_parta.pdf