Hi! I have a lightweight canvas trunk cover that will fit a 2016-2018 Prius' trunk (it came with the car when I bought my car used). It has looped cords on the sides to hook into your trunk's sides. And guess what? If your trunk doesn't have the hooks on the sides, then I have the four hooks for it - you just pop them in the current spaces for it, or have a handyman have the trunk's sides fitted for it (it's easy to do). Basically, the trunk of these Prius years come with the hard cover retractable one as a default but you can outfit the trunk to have the soft one instead. Please note that this cover does not fit a Prius C, V, or Prime.

Here is the website if you want to see the specifications:
https://parts.frontiertoyota.com/oem-parts/toyota-tonneau-cover-soft-lightw...

Toyota Prius soft trunk cover (Near Santana Row)