Hi, First of all welcome to the 'bolt!
I'm not too familiar with the clamshell door but I will assume the glass is flat.
If so any competent glass shop should be able to cut one for you using laminated glass. If you prefered tempered glass they should be able to get one made for you.
Maybe someone will chime in and confirm if the glass is flat or not.
Good luck with your project.
Jock

1950 Chevy Advance Design 3100 in Scotland
In the Stovebolt Gallery
More pix on Flickr.
I've definately got this truck thing in my blood ... my DNA sequence has torque settings
"Of all the small nations of this earth,perhaps only the ancient Greeks surpass the Scots in their contribution to mankind" Winston Churchill.