John
I am afraid you will be disappointed. Although it looks like there is plenty of room you have to allow for the suspension travel, Incursion of the shock absorbers, the differential etc. etc. I have replaced the tray on my roadster having been through four on two cars. Mine is built from aluminium by a local sheet metal works to a pattern I produced from cardboard. True to say though that you can create a much more useful space.
See photo's here:
http://www.talkmorgan.com/ubbthreads.php/topics/95138/Roadster_Rear_Shelf_Storage#Post95138
