I bought this liner awhile back so I could transport my two huskies. I wanted something that would protect the plastic interior pieces as well as the carpet on the trunk lid and the back of the rear seats as well as the floor. There are so many crappy ones out there. I bought this one with a lot of reservation but I was so pleased with it.
The liner has velcro sewn to it. It comes with the hook side of the velcro sewn to the liner and then has pieces of the loop side of the velcro with a strong adhesive, basically you just install the liner and get things lined up and then start peeling the backing of the loop side and adhering it to the plastic trim pieces in the rear. Some of the velcro pieces simply velcro to the carpet like on the floor and on the back of the rear seats. Basically when you get it all in you have a fitted liner that is crazy durable. It features a zipper for the rear folding seats, so you can still fold them down or fold only one down. You can also remove it and throw it in the washing machine to clean it should it get real dirty, i did this a couple of times. I had this in the car for about 3 years and it did its job. It is still in great condition. I have since sold the RX300, but I held onto this liner. Below is the link to the website where I bought it. Its also good if you just want to put dirty stuff in the back of your RX and not worry about ruining the carpet or scratching the plastic trim pieces. I paid somewhere around $280 for this. I'm looking to get $60 shipped anywhere in continental US. I would put up pictures of it installed but I don't have any. The stock photo is a pretty accurate representation. You won't be dissapointed with this, I promise.