I found what I think to be a fool proof method.......
With the side panel temporarily fitted in place (just sat in place with some clamp holding it rigid) you can simply reach inside the rear and main driver chamber and draw exactly where the panels are onto the inside panel....simple...it just takes a pencil, no large sheets of drawing paper to trace the lines....
Remove the panel and then add a second line to show the thickness of the internal panels and its done, very accurate and very quick....
Dave
edit...just spotted this was noted at the end of the very thread I was talking about...and here was me thinking I was a genius....

http://billfitzmaurice.com/phpBB2/viewtopic.php?t=1508