An oversize fanny pack is the way to go. I'm currently testing a big one from MEC (6 litres of storage with external water bottle carriers). ...
If money is not a too big problem, try a KIFARU Marauder (about 40 liters)with a detachable E&E pocket (about 16 liters):
Marauder pack E&E back pouch (usual disclaimer : I'm not affiliated with KIFARU and have no economical interest in that company. On the opposite : I'm starting to spend too much money on their products... help !!!

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