Well, "ask and ye shall receive", James Black has come to my/our rescue with two plans of this type of roundhouse and a series of photos of the Cowra loco shed taken in 2005 that show the lights between the last track and the side wall!
Thank you James, this information is very much appreciated and I am sure that there will be others out there who will be able to make great use of it.
Here are links to the two plans, click on the magnifying glass between the rotate icons to get a high resolution plan and then right click and click on Save Picture As to save to your own PC.
And here is the link to the photos of Cowra roundhouse in 2005.
Photos Untitled24 and Untitled25 show the side lights suspended from a beam that is between the wall truss and the next truss towards the track in the shed.
Another detail I notice is that the timbers aren't black as I would expect but more a dark reddish colour with an 'overspray' of black, just in time for my spraying.
Now James has caused me another problem! A couple of photos show the belt drive for lathes, etc. mounted on one end wall. I think I might draw the line now or I will never finish the shed (this wall will not be easily visible on the layout).
Thanks again James.