unfortunatly building a wall is never the same thing twice.
So you can't just say 40x30x36 double thick and go.
Sometimes the contour of the space for it to go in is just not forgiving and you have to take extra time just to get a good flat spot just to start in.
So I can't just give you a formula and go at it.
basically step one......figure out how close you want it.
Get in the scion and pimp lean the seats back to hand job position.
and put the wall right behind that.

...might as well right?
Why sacrifice the sexy time?
So after that you have to make the "dance floor" ...that has to be rock solid and capable of being drilled and bolted from the chassis up to the bottom floor of the wall enclosure.
DO NOT HALF ASS THIS.....
A wall can come at you if not bolted down...
A collision at 40 mph may not kill you ...but it will if the wall comes unattached and chases you into the steering wheel.
Then its back to construction.... 2x thick on everything and make a baffle as thick as you can..... 4x minimum....sky is the limit....and then you can either leave the back wall 2x or you can 2x and use 2x4's...2x4s are light and rigid and add a ton of support to the back wall.
Put 45degree angle peices in the corners.
Glass if you want too..
Round off the edges...
All thread the box to decrease flex ... flex = loss of spl waste of power and performance. When using allthread....use washers and 2 nuts together on both sides of wood...this is called JAMNUTTING...the first nut holds place and the 2nd nut jams the other one so it doesn't move after the funk is applied.
The it is always a good idea to allthread from back wall to somewhere behind the box to help keep it stable incase of an accident.
I cannot stress this enough.
95honda foamed his wall in...that is basically fool proof...but 85% of people that try to do that will screw up and bulge their bulkheads out and have a big goofy looking bulged panel car from then on.
the rest is basically up to you....that is the basic layout of a good box...construction iand safety is what will make or break you.
If you suck at building, not amount of advise i give is going to be worth a crap.
take pics along the way and we can guide you to the right direction.
Also..if you do want to glass it...gravity is your friend.
Build the enclosure outside of the car....
And glass out in the open air ...being able to lay the enclosure on the sides you are glassing is invaluable.