I second, third, or fourth, Glycerine!
A long time ago when I worked on fishing boats, I did a lot of sea snake skins with the stuff, and about 25 years ago did a rattle snake skin for a hat band. I still have the hat, and the skin is still soft and supple with no scale loss.
I didn't stretch the skin, instead put the glycerine on it and rolled it up. I did that for 4 or 5 days until the glycerine wouldn't soak in any longer.
John