Hi
@1BadTundra thanks so much for notifying of this add-on. I'm so glad I got this - in fact i assumed it was lined back there until I saw this post and realized it was "unfinished". I hope this helps but the weather stripping on my 4BW was enough to wrap over the edge of this cover all the way - it looks seamless and gives a bit of extra hold on top of the clips (that also hold quite well... have to make sure clips are fully clipped in). I havent used any tape or anything else. I even took the ride out for some good sessions wondering if it would flex or bounce out anywhere but it held solid.
Its hard to explain until you get in there but all 4 sides in the trunk lining there are discreet places where you can overlap or blend the liners together where they provide support for everything together.
Please feel free to DM or anything if I can help!
Toughest part of this install was Cirque Du Soleil'ing my fat arse back there to secure everything properly... Lol
*also I noticed someone ask, I did find it made the cabin a bit quieter after install. I didnt mind this as I still hear all the burbles and belches, but just what I observed with my car post install.