Nice work so far Ross..
Mild or stainless steel are favorite materials. 1.6mm thickness sounds ideal.
Not sure I would trust ally myself, but it might work?
It will be nice and soft and easy to bend as a pattern, so nothing lost.
Bear in mind that the 5 speed 750 cases differ from the 6 speed 900's, so be prepared to make some alterations as my pattern is for a 900.
I am given to understand that it's a bit tighter inside the 750 and more so on the 600, so if you find that the pattern needs some significant changes please post them on here if you can find something that works.
You may have also noticed that on Dirty's 'saver on an earlier post here, that his tabs are staggered...So something else to look out for.
Good luck...
(P.S. Just noticed that this thread has become sticky...Cheers Rob?)