My suggestion would be to buy a flat bottom rail and have any ol' machinist cut a square slot length ways in it that touches the high spot of your action at the screw holes as well as both corners of the cut...I've done this for myself quite a bit and is easily done and just as sturdy as a full contour cut
I am looking for a supplier of steel blank rail stock in Canada. I would farm out desired mill work as well if a one stop deal.
If you have a buddy with access to a nice CNC mill, just have him order 4140 key stock and have it cut to what ever you want. Once the initial program is developed for cutting the slots and such, altering the radius on the bottom and where the screw holes go is pretty minimal.
I have gotten custom steel rails from atrs and K&S. True one offs for oddball guns. Also had egw in the US do a 3ft long custom length of picatinny rail milled for a rifle, for 75$. Called them and they took the order over the phone.
Check this guy out. He's a member here, but I can't recall his username.
https://www.prprecisionrifles.com/products-1
On the Tikka rails, he states stainless or 4140. I never opened any of the others, but it might be worth asking about custom rails.