Fabricator, I take it you donrt want to mess with the look of the bike much.
A few things..
Pretty well any sort of bags, need some kind of frame/shelf or combo of.
Sissy bar, tall or short with a bit of rack behind it works well, if you can stand the look or have passengers. I am running one and put a shelf bebind it and run a (good sized) leather saddlebag.
This might be best for you.
Build a clean looking rack outa stainless that is behind the seat and as wide as you can stand to look at. Then get a sports type bag with top zip, that goes across the rack. The bag will need a bottom stiffener or you can make a piece that fastens across the rack, or swings out to support the bag.. that can come off when you remove bag, leaving a clean-looking rack.
I love Alu, but unless its plated , or coated, or anodized, its gonna turn stuff grey..