I find this subject sad, in a way.

Technically I think there's a very good case for doing something simple. Freeze practically any tissue sample, and someone at a later time can extract tiny fragments and extract the DNA using the well-known polymerisation techniques. With multiple cells and tries, you can exactly sequence the entire genome. We probably have the technology already to do this, and end up uploading these species to the Internet. And at some later time we'll be able to rebuild the sequence, put it into some suitable cells, and fire off a pregnancy.

That's not the sad part. The sad part is that the pleistocene is gone forever, and that's where all these animals belong. That kind of Earth is passing away, and is never ever coming back. The arrival of technology is going to affect the Earth as profoundly as the advent of multicellular life. What's happening now is merely the very very beginning of this change.

You can preserve the rhino if you like, but its habitat has already gone, and the finality of that will become clearer with every passing year.