Sorry for going MIA on my own thread!
Indeed, I could just add some length to a pattern made from my medium. However, I figure there might be other differences between the medium and large, and I'd be interested to see what those were.
What is the relation between the GVP pack(s) and the Jam2, again?
As far as material, I'm not particularly concerned about matching that, per se. (In fact, I'd probably be making a more durable version.)
John (fellow Cascadian!): thanks for the offer, but I'm going to DIM if I do that.
Thanks for the ideas.