Well, it ended up not taking as much time as I thought it would take
A customised giant rando bag with a zip-out lining. Fortunately after a few attempts to pin the zipper in place I threw in the towel and bought some water-soluble tape to pin the zipper to the bag and the lining. What would have been a many-hour job turned into about 45 minutes after I was able to actually keep the bias tape and zipper in place while sewing it together on the mighty Pfaff 130.