I've been thinking of building a FG enclosure back there since I bought my car in 2012... never got around to it though, as it will be a ridiculous amount of work for an amateur like me.
Now the $2000 estimate I saw several posts back seems extremely excessive. I'd guess somewhere around $1k for a nice one-off custom enclosure for that space. If someone were to plan a GB for them, they'd probably come down to around $600 ea - but shipping would be really high, pushing the overall cost well above what the vast majority would likely be willing pay. On top of that, the enclosures for the trunk area already offered will almost certainly produce noticeably better sound.
I still might build one (after I finish a few other big projects). If/when I do, I will post it up here, and if there is enough demand, I might consider making a mold so that it could be duplicated. I have to admit, I'm not a pro at this sort of thing, so if I made a few, I couldn't promise high-end, show quality results.
Here's a simple FG enclosure I built to squeeze a 10" sub behind the passenger seat my MR2 without losing any passenger seat legroom or adjustability:
