I know Drawabox is not the usual place for anatomy discussions. I suggest going to Proko on Youtube. [https://www.youtube.com/watch?v=PWGRsD0xen0] He builds the pelvis as a bucket with some cut outs. The pelvis is essentially hollow though. If you are looking to study anatomy, the Proko course on Anatomy is good. It is not free and it is not cheap, but it is exhaustive in content. Something like 75 hours of video to watch with homework assignments. This is not an endorsement. No affiliation.