Thats absolutly, total Madness.....
and I LOVE IT!!!!
I cant wait to see this get started.
A few comments on your idea:
Im not exactly a professional metallurgist, but could you make the back pack by making a wooden model of the rough shape of the back pack. Then get some of that aluminium (though brass is softer; but cost!!) and hammer it into shape so it fits around the wooden model. Then do the same for the other side of the back pack so there is a front and a back part.
The if it is too flimsy, fill it inside space with some foam, even if its that expanding stuff you can get for holes in walls. Then attach the two part together.
Also, I would get some drawings worked out for the model to get the proportions correct.
For the Purity seals, use a watercolour canvas material. Its strong an flexible IIRC.
And finally, just contact GW and ask if its ok to do this. I know they will probably say yes, but you dont want to be half way through and get a Cease & Desist order in the post from GW when they find out.
Anything else I can do to help, just ask.