Quick update:
I sold my McIntosh sub a few weeks ago, so to keep the boot looking decent I taped the stock sub Cover/Grille on with double sided tape (the panel with the mounting holes was removed). But that kept falling off despite using 3M tape.
So I decided to bite the bullet and install the new sub box. Just bolting it in place, no wiring yet. Took a few hours but finally got it done
This is the install guide for the JL Audio Stealthbox. It helps a bit with this WickedCAS install - since that doesn't come with instructions:
JL Audio Stealhbox Install Guide for Legacy WagonThe JL Audio ones come with a grub screw to secure the sub box, the WickedCAS one does not. The JL Audio ones also come with some wax, the WickedCAS one does not. The wax is there to help you make an imprint of exactly where the screw hits the metal. That's where you drill into the rear metal panel - after removing the RH rear tail-light.
I did all the preparation for that, but wasn't keen on drilling into my car. So just before I started drilling, I thought it looked like the WickedCAS box had an imprint of where the bolt should go. That basically lined up with the existing cargo-net mounting bolt position.
BINGO I'll just use that bolt hole as a mounting point . Turns out the imprint was probably just where they vacuum formed the resin (was about 5cm off), but either way I used this method to secure the sub box. And it worked well. No drilling required. I got a longer bolt and used the wax method (in my case just with blutac) to get the exact hole position. And drilled into the sub box there - worked perfectly!A washer underneath, and the sub box is secured!
Oh and I also had to trim my Subaru boot mat, since it has a lip on it that would've totally gotten in the way of the sub sitting flat.
And as you can see, I painted the back of the sub box black. The paint didn't interact well with whatever excess resin was on the sub (bubbled a bit). But a few more coats and it looked fine.
Sorry, no final picture - was getting too dark. But looks exactly like the picture with it temporarily sitting in place .