I purchased my FF from Vintage Trailer Supply (Fan-Tastic Fan Vent
) and got the curved inside trim. It fits and looks good. The first trim piece came and there was a crack in it. I called Steve at VTS and they shipped a replacement the next day at no charge. Can't beat that for service. Also, they have the option of a gray top. Most folks don't notice the difference in it and the 2 old style aluminum vents on my Trade Wind. White would look out of place.
A couple of tips on the installation, don't use the supplied screws. Instead gets some #8 stainless steel sheet metal screws and small SS washers. The mounting flange is counter sunk and when you use the supplied screws it puts a lot of pressure on a very small area, it is a crack just waiting to happen. By using pan head sheet metal screws and a small washer the clamping force is spread over a much greater area and you won't crack the flange. ( This is a Colin Hyde tip and he has done hundreds). Don't use the foam gasket, just a good bead of Tempro or what ever sealant you use (not silicone) and set the fan into place. Screw it down and then put a small dab on each screw head. You will have to enlarge the original hole and square it off. A template is in the box with the fan. You don't have to cut much. I used a 4in angle grinder with an aluminum specific disk since I did not have a high speed air cut-off tool. It worked great.
Final thought, the rain sensor is worth is weight in dry towels!!! You can go away for the day and leave it open to cool things and if a thunder boomer comes along and you are not back, it will save the day. It has for use several time.