I have the same car and it did NOT come with a trunk cover. However, I found this one works great: Plus I think if you FX did not come ordered from the factory with a trunk cover then you will not have the side cut-outs or hardware to mount an OEM cover if you find one. So your best bet is to go with this option, which works great for me and you don't need any hardware, because if fits tight between those small rear window cutouts without any modifications.
They call it a “Retractable Rear Trunk Cover Security Shade Shield” and it only costs $72. Here’s the website link:
(Tip: Don't use the straps. You will not be able to put the seats down if you do and you really don't need to anchor it anyway, because the width of this cover sits in place really nice. And I use a S-Hook to latch it to the tire hangle in the floorboard to hold the cover in place.)
** And everyone should carry one of these IMO:
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