Half the battle in online searching is knowing what something is called.
I contacted Freeman Wax Co I mentioned polymer based clays used in the auto industry and the rep says they have it. Its called Chavant clay.
http://www.freemansupply.com/ChavantIndustrialS.htm
Also look here:
http://www.sculpturetools.com/chavan...sstandard.html
On this later page look under descriptions for J-525 and CM-70 and you will see that they are suitable for milling purposes. They vary in hardness at room temperature. You may want to contact the companies directly to decide which version would work best.
Alternatively, you can make your own. Let me know if you want to do this and I will dig up a sheet on the various additives used. Be forwarned that this will require testing.
I hadn't even thought about Bondo and it may work for your application. The reason I would go with clay is because it can be reused, and if you screw up in applying it to your model, it can be resoftened with heat and reapplied (unlike Bondo).