I've been waiting for somebody to finally do this Peter, and you are the right man for the job. Unlike me you've got the skills and balls to make up parts as needed, so this should be good!
How about a KTM 690 subframe tank? You are never going to be able to build anything as light, with the same capacity and as crash proof with steel and aluminum.
If you build the attachments to the main frame strong enough and replace the tank bolts, it will have plenty of luggage carrying capacity as well. Just don't go nuts with heavy hard panniers, but you probably wouldn't do that on a more dirt oriented bike anyway.