I have made a couple of "molds" from parts downloaded from thingiverse, I am not sure if you are familiar with OpenScad, but with openscad you can import the stl file, create a cube around the imported part and use the difference() function to subtract the "part" from the block, which will give you a hollow cube with the part shape in it.

That easy, eh? I may have to try that!

 

yeah it is that easy

Here is an example;

I downloaded the exploitable heart from thingiverse,
I imported it into openscad, created a cube about half the size of the heart and subtracted the heart from the cube and there you have 1/2 of the heart mold, print 2 and you have a whole mold, or you can create another cube to make the other half of the mold...

 

on a side note, whenever I need to come up with some "new" material for rubber rollers or hard plastics I get my polymers from this company >> http://www.innovative-polymers.com/, they may have some UV curable resins or some other polymers that can be used for your purpose...