We have been using a product called Polyjel. I think it's the same stuff dentists use for dental impressions. It comes in a tube like toothpaste. When this stuff cures after a few minutes, is has a rubbery texturel to it. It reamins flexable to it is easy to get out of tight spots, and doesn't stick. But stays in tact so you can get accurate measurements. It's is real easy to cut too, so cross sectioning is easy. Before we started using the Polyjel, we used plaster, which was messy, and brittle, so keeping a casting in tact was difficult.
Jason