There is a problem with doing that. There is a flat calibration plate on the sonic testers. If you make your probe curved can you calibrate the instrument ?
We have a wiz kid electronics expert at work. He can fix any fixable electronics device put in front of him. Best I have ever seen in my decades of maintenance work. Home schooled, never been to a school in his life and knows more about electronics than anyone I have ever seen. One of those guys.
He is the one who first fired up both my sonic testers. Handed it to him and told him to nerd out on it which he did.
Went and got different types and shapes of metal, steel, iron, brass. Flat and round stock with calipers in hand to check accuracy.
It was accurate on everything we put it on.
Even on the machined 1/4" wall steel cylinder, 3 1/2" inside diameter it showed .249 . That is smaller than a Pontiac 4.12 bore by a long shot and it measured the steel accurately.
I have to believe it would measure iron the same. I need to test it on the kind of iron sleeve you press into a engine block, that would tell you for sure.
When I asked him about sanding it to fit the curve of the bore he was against doing that. Said, how are you going to calibrate it ?
The one I bought
https://www.ebay.com/itm/314314770567
the add says can be used on all kinds of pipeline and those are not flat. The instructions say it can be used on pressure vessels , boilers, tanks ect and those are many times round. Says nothing about only testing flat stock.