I picked up an old Necchi 523 workhorse sewing machine for this very purpose. My first project will be making a bag to hold a generic 5A-hr battery pack for an old Schwinn Mod headlight that originally came with a holder for 4 Alkaline D-cells.
After looking into it, sewing something like this up isn't all that hard.
I'll then be making a bag better suited to attaching a Marwi Elite battery pack to the frame. A few cable swaps and some battery pack shrink tube later, the Marwi will be equipped with the 5A-hr pack for its 12W spot beam and the Schwinn will have a tiny 4A-hr pack for its 10W intermediate beam.
I might even move on to making a custom trunk bag for the Blackburn Crossrack.