I decided to try to print a mold in which I could cast a two-part silicone rubber.  I chose the octopus model that so many people print off of Thingiverse.

At first, I simply subtracted the octopus STL from a block, using OpenSCAD.  The print which resulted was a decent mold, but the octopus has a protruding body, which created an undercut in the mold. I resolved this by restructuring the mold into two parts, with the dividing line around the maximal body bulge.  I also added a small open spot at the top, to allow me to push the molded part out of the mold, rather than trying to pull it out.

This is what the two mold pieces looked like, fresh off the printer:

Notice the four holes in the bottom, with pegs protruding from the top part.  This allowed me to get perfect registration between the top and the bottom.  This is what the assembled mold looks like.  If you look closely, you can see the pegs filling the registration holes.

And this is what the rubber octopus looks like after the casting operation.  It feels as if it were made out of rubber-band rubber.  The flat spot on the top is where I pushed to get the octopus out of the mold.

The whole process was both quick and fun!

Later – I guess I got carried away!  I discovered that if I squeegeed the excess liquid rubber off before it set (see mold on the left), I did not have to trim the tentacles out separately.