I have a jeep and there's a thing with giving/displaying a rubber duck in the car (usually on the dashboard). But I have never seen any mounting platform for these ducks anywhere (aside from etzy). You can use a double sided tape on the bottom and mount any rubber duck on its prong (should be a same shape/size as the squicky whistle, a little tight if any).

As this will be inside your car, it should be at lease a PETG or any other high temp resistant material.