I know this may sound self-explanatory and fairly simple, which it is, but I like to give people ideas and help maybe they didn't have or think of.
I love the OEM Mirage hard plastic/rubber trunk mat or even better yet, the Weathertech one, but can't bring myself to spend the amount of $ on it they ask for. So I purchased o a universal rubber mat off Ebay (or Amazon) and cut it and installed it in substitute for those. It cost a huge $16.80 shipped to my door.
Universal Rubber Trunk Mat
Black or any color marker that you can see once written on the mat
Trunk OEM carpet or accessory Mitsubishi carpet mat (which I used)
Here's the mat that I purchased, from Ebay.
Lay out your rubber mat on the floor, with the pointy knobs underneath.
Take the carpet or mat out of the trunk and lay it on top. Trace around it with a marker (i added approximately 1/2" to the perimeter size) and cut the pattern out.
This is what it should look after cutting it out. The ripples at the top are from being rolled up. They'll lay flat over time, plus mine tucked under the rear seats.
Re-install the existing carpet or mat, then place the rubber mat over the top. I lined it up with the rear of the car first then tucked the front of the mat under the rear seats. I am happy with how it turned out, and now it will be easy to clean if need be.