Rear Door Speaker Upgrade 6x9? 8"? '99 Forester

I just put 6.5" speakers in the rear doors. I had to do minimal cutting to get those in. While I was in there I noticed there appears to be a big hole that could house a 6x9 or maybe a 8" subwoofer. I rolled the window all the way down and it looks like the speaker could clear the glass if the magnet is not too big or perhaps with spacers installed. Of course the door panels would have to be cut out and a nice grill or cloth cover fabricated. Has anyone ever done this? It is appealing to me because I want deep bass but do not want to lose my cargo area in the back hatch area.