Thanks for the update. Please keep us informed.

I just finished my full undertray this afternoon. My cat is immediately aft of the 90ish degree bend in the pipe where it comes down from the manifold and bends underneath the car.

I left an opening in the coroplast there, but enclosed everything aft of it using the basjoos aluminum bridge (TM) + tape technique.

I also have a remote temp sensor (though it only goes up to 50C) which I plan to insert into the undertray in various spots to see what's going on while I drive.