I know this won't help you but I made a couple fillet knives years ago from 440-C and they warped, those were heat treated by me. I kind of just quit making fillet knives at that point. flash forward 25 years...now I use aluminum blocks for air cooled stainless, everything that comes out of those blocks is dead flat, I also heat treat the profiled knife only, fillet's only because their long and thin, and put the grind on them after heat treat dipping in water frequently so as to not ruin the temper. I know this won't help you if your sending them out but it may in the future if you get an oven.
for my fillet knives I've been using 12C27, it's clean and easy to work and I heat treat them to 59-60. I've had some good feed back from people here in Fl. that used them on redfish. I'm still in the test mode of this steel but so far I like it.