Dan, are you saying that as you forge the lateral crown in the plate, it takes on a crescent curve at the same time? I want to take a crack at making a two piece forged butt plate and trigger guard for a southern rifle as well, and this is all good advice.
We wer forming the "crown" on the buttplate using a piece of shafting as I recall and it forming it around the shaft hot it just formed the crescent at the same time
I had been doing it differently previously but decided they would look better with more crown.
Partly from holding it with tongs I am sure but it did bend to a pretty nice crescent I did another for a TD I made into a Gemmer type and it worked the same way.
These are not that tough to make.
Make a paper pattern of each piece and make sure the will make what you want.
John Baird wrote a series of articles in Muzzle Blasts on Hawken rifles and one had a illustration of the two pieces needed to make a sheet iron buttplate.
Dan