Materials usually used in dorsals fins specifically in the ATR 72
The loads on section of a dorsal fin are minor, just from the dynamic pressure of airflow that may be striking the panel from the side during yaw excursions in turbulence.  It's a &quot;lifting&quot; surface, insofar as it's contributing to the restorative yaw moment being part of a stabilizing fin, but the loads aren't super high and it may even just be attached like a fairing using dzus fasteners or camlocks or  similar if there is something inside that needs ready access.
Can't speak for the ATR specifically, but the material is probably either 2024 Alclad sheet like the adjacent structure, or if the part has compound curves, you might see a more formable alloy like 6061 or even 5052 in a non-structural part like that (you will see 6-series alloys in heated leading edges for example).  On newer designs it's more likely to be composite, usually Kevlar or a fibreglass/Kevlar blend (fibreglass is better in compression being stiffer, so it is sometimes mixed in with the Kevlar) and most recently carbon fiber.
If I was tasked with redesigning the part to a new material, it's likely because it's aluminum and the goal is weight saving, and I would likely choose carbon fiber as the replacement.  The optimum would be carbon fiber skins sandwiched over a nomex honeycomb core, the way interior floor panels are made.
An important caveat about using CF, is you cannot let it live right beside aluminum where there is a potential for direct contact. They are well apart on the galvanic scale and if there is water present the carbon will destroy the aluminum. If that's a problem it might be better to go with Kevlar composite.