Just got a trunk liner from WeatherTech - it fits the Clarity PHEV great, hump and all. The pictures on the website are completely wrong, but the product really does fit. I'm not getting any promotional kickback, I'm just a happy customer. $120+shipping. Here is the link:
https://www.weathertech.com/honda/2018/clarity-plug-in-hybrid/cargo-liner/
https://www.weathertech.com/honda/2018/clarity-plug-in-hybrid/cargo-liner/