I've finally done all the channels on one side of my stays. Of course, now it's second-sock syndrome and doing the other seems like a kind of torture, but still - halfway there.
I originally cut the front too high - or the rest of it too low (but I'm short-waisted so it worked out all right for me). Not sure how it happened. But it was an easy fix! These don't have a lot of boning, unlike the stays most people make, but these are a reproduction of a pair at Cherry Hill, so I'm pretty sure it's period. The curved channels ought to straighten out and overlap with the addition of boning, but I might need to take out the stitching and make the curve more gradual.
The construction method's a bit different and I'm going to do a post on it later. I think it works pretty well!