As it turns out I couldn't unwarp the scales. I clamped them to a brass bar and steamed them for 1 hour, the dried at 200 degrees f for 2 hours. After cooling, they returned to their warped shape.

I then glued them to a piece of ivory paper micarta, clamping them tightly. This took out a lot of the warp. Next I flat sanded as much of the micarta off as I could. This left me with a flat micarta liner inside of the mammoth.

At Office Depot I found a display with Sharpie permanent markers in a wide variety of colors. I used these to color the micarta to match very closley the color of the mammoth. The scales are a little thicker than I would like, but at least they close cleanly now.

Thanks to all for your advice

Bob @ OCD Razors