We were looking for a way to make a boggy uterus. We blew up a glove with air, tied it, and put it under the abdomen of our sim. It worked beautifully!
I like to use one of those gel filled freezer packs. It is very squishy and works well.
For a boggy uterus, I bought a small soft Nerf ball and cut it in two (use a serrated bread knife). I also bought a small Nerf basketball for a contracted uterus. I then glued the bottom halves of small easter eggs to the bottom for the pelvic bones. I laid some soft fleece on the top for skin, and voila! This way the OB instructors can help the students differentiate the feeling of a boggy vs. contracted uterus.