MG88

One thing I've done is put layers of magnet tape on the window hatch and the steel of the hatch to "prestress" the lower corners of the glass. I now have to press slightly to get the glass closed, not sure if it helped volume, but it should reduce the window resonating. You could also check the latch to make sure it hasn't come unglued from the window, I had to re-glue mine.

I was told that rubberized undercoating works good to seal the box better. Not positive it helped anything but I thought it was worth a shot. I also added a wood dowel in a weak spot, it has a very slight interference fit and is glued in. Some use silver HVAC tape to smooth out airflow and seal things up better.

I think the rough design you made could be done and sound very similar to a "more square" box. I'd add something like recycled denim insulation to the back of the box(inside) to attenuate the backwave so it's not fighting itself if the mounting depth is tight. Just make sure the pole vent can breathe. My setup is something similar (shallow mount), it sounds better with some denim insulation behind the sub, it plays lower too.