I tried something on my last build that worked very well.
I took my buttplate and taped it to a small engineers square, to make it able to stand on it's edge. Then I scanned the buttplate edge on my scanner. I took the image of one side and made a duplicate which I flipped in a photo program. Print out both images and you have a actual accurate contour of your buttplate. Apply some double side tape to the back and adhere to your butt stock and saw away. Very easy and accurate and minimal final inletting adjustment required.