Is it essential that the sail fits in the groove or can it be loose footed? Many mainsails are loose footed these days, including the Elvstrom (so called Hi Tech) fully battened main on Playtime.
I have a C&J sailcover (similar to the Pack-a-Main I guess) that fits in the groove.
Try leaving the foot loose, connect the clew to the outhaul, put a bit of tension on the outhaul and see how it sets.
Roger