You can cheat a little here if you dont mind a front toughened screen (most cars used to have this)
I seem to recall custom made toughened screens didn't cost the earth provided they are flat (the last time I looked, a few years ago).
Assuming you go for the flat option, the screen can be mounted in the square(ish) hole in the chassis by means of four lugs welded to the vertical tubes and the screen can be bolted to the chassis, leaving 5 or 6mm clearance (like ford ka rear quarter glass). this would mean having four holes made in the screen and mounting it using rubber grommets.
With regards to the wiper, use a metro rear wiper motor, this can be bolted to the screen (another three holes in the glass). Its quite a neat unit and lends itself well to the job, provided you dont mind a one speed motor.
Did this years ago on a car I made and used a company called solaglass (not sure if they are still around)
I just provided them with a paper template of the glass shape and where I wanted the holes and they made me the screen in 6mm toughened glass.
The Talbot sunbeam had a similar setup on the rear screen. (no old jokes please Im only 38)
Bolting a laminated screen wouldn't be able to cope with the stresses of chassis flex, never mind putting a wiper in it.
Can't remember the exact costs involved but £170 seems to ring a bell.
Toughened glass no use for the IVA !!
But once you pass...!
Jamie