Hi John!
I am just getting caught up on a backlog of messages.....
The Safety Screen that is used as part of the demonstration unit for the AAW Symposia for the last two years is made from 1/4" thick polycarbonate plastic (Lexan is one of the trademarks associated with this type of plastic). The vertical and horizontal dimensions are not critical, but should be sufficient to cover the "line of fire' near the headstock, as well as a bit down he lathe bed to deflect chips generated during turning. The size of the AAW screens was dictated somewhat by the limits on the extruded parts of the structure, and the ability to have these units break-down for shipping, so in your application, the size of the clear polycarbonate screen can be chosen to accommodate the size of your support system.
If you can look at the June 2012 issue of American Woodturner, there is an article by Deryl Duer which explains the general set-up and design of the units which are assembled from extruded aluminum pieces, fittings and quick connect bolts, etc.
Deryl Duer (Franklin, Kentucky) was the main architect for these units and you might try to contact him for details - I believe his information is available in the Member's Area (log-in first), Click on 'Member Directory' and type-in his name.
I'll make a note to try to get these plans put-up on the web site under 'Resources'.... as soon as we can.
I hope this helps!
Rob Wallace