I'm have a custom, hand made subwoofer enclosure that has been in my 2007 Outback for about 18 months.
The box is fitted for the left hand side. I did this for directionality of sound to the driver, and because it meant I didn't have to remove the cubby hole/storage space that was on the right hand side.
The required modification involves cutting a piece out of the rear trim, to expose a cavity, which part of the box slides into. I used an mdf base, front and top, while all the rest of it is fibreglass to take up all the available space. The fibreglass back half was screwed and fibreglassed to the mdf. It has had deadening paint applied multiple coats inside and out, and then has felt glued over all the visible sides (there's also sound deadening panels on the back sides). It looks as if it were part of the original fit out.
It is a sealed enclosure (not ported). I have it running with a morel ultimo SC 10 sub (so its designed to fit a 10in sub). It gives loud impactful and tight punchy (fast) base, without being drony or otherwise.
I'm happy to sell the sub and the enclosure separate or together. The sub cost me $500 when new.
The enclosure took at least 40 hours of work to build. I had a couple of quotes from stereo places getting on close to $1k just to do it in timber, not fibreglass (which would have meant much less volume and/or taking up boot space), so built my own. When installed it takes away very little of the volume in the boot space, basically has no impact on what can go in the back. This was the main reason I went to all the trouble. It was a huge effort so kinda sad to see it go, but want to make sure it gets used by someone else, too much work went into it to not.
I also have a rear floor mat mat that has been trimmed to suit, which I'd be keen to swap for one that isn't trimmed, for when I sell the car. I'm not completely sure how to swap the plastic panel that I cut, but I could possibly swap that with an uncut one as part of the deal (would save you cutting yours, and I can sell mine unmodified!)
I also have the single din A/C unit and fascia plate, which I'll list separately, which I imported from Japan so I could run an aftermarket head unit.
Can post more pictures if someone is genuinely interested. Was thinking maybe around $375 for the sub, and $600 for the enclosure. I'm located in Canberra, but could box it and have it shipped via courier.