Transition molding/casting materials?

Now here's a topic I don't hear much about! Once again I think I've gone just about as far as I can go with my clay sculpting. Just about everything I do at this point in the process seems to yield little to no improvement. The clay is just too malleable, and I end up doing a good deal of tertiary damage that I end up needing to fix. So, I'm considering doing a transition mold, but I don't really know what to use for it.

I want something that is easier and quicker to work with than silicone and also hopefully I'll be able to save my "good" (more expensive) molding and casting material for a final product that requires a lot less cleanup. Obviously it doesn't need a lot of longevity, but it does have to accurately reproduce what I've done so far.

And what about the transition cast? I'd really like to work with something harder than clay that can be sanded and worked with harder tools, and can still be filled in with bondo or epoxy clay or some other kind of compound that will harden to a point that I can work it more effectively.

So, any ideas? Thanks much!!

dont know about the moulding material but plaster of paris is good for cheap casting if all you want to do is sand and fill, makes a BIG mess when sanding tho