I couldn't find one that would fit properly, so I made one from black HDPE I bought from Graingers.  http://www.grainger.com/product/Sheet-Stck-1ZAZ8?functionCode=P2IDP2PCP.  Buy more than you think you will need, to cover the possibility of mistakes, and also because once you see how much a frame sheet improves a pack, you will want to make more of them.

It's a fairly complex job for this particular pack, because there's nary a straight line in the whole sheet frame.  You will want to make a good template, which should be a single piece of thin, but sturdy cardboard, about the same thickness as the HDPE material.  Art stores sell the stuff in sufficiently large sheets.

Begin by placing a piece of masking tape down the center of the pack, and drawing a line on it which runs down the exact center of the compartment which will house the frame sheet.  This compartment is pretty obvious, the outline being visible from the ventral side of the pack.  Mark the vertical, centerline in 1" increments, top to bottom.  Measure from the center to the side of the compartment at each interval, and transfer the marks, representing the side of the compartment, to your template, on which you have previously made a centerline just like the one on the pack.  The measurements should be equal for each side at each increment, which confirms that you have the correct centerline.   You will need to free-hand the outline of the template between the marks delineating the edges.  The tops and bottom of the template can be marked out working from the centerline as well.  Once you have your template made, try it for fit, trimming excess material, and marking on the template where more material should be added.  Make and try out a second template, correcting the mistakes made on the first.  It should fit much better than your first try.  Make a third if you feel it necessary.  Once you are satisfied with the fit of the template which should be just a tinch smaller than the compartment, transfer the outline of the template to the HDPE sheet.  Cut the sheet using heavy-duty shears, and polish all the edges using sandpaper so that the HDPE material has NO sharp corners or edges anywhere, which could cause the fabric to wear, and possibly fray.

Any small pack can really benefit from the addition of a frame sheet, and the result will surprise you at how much benefit it gives.  FWIW, this particular pack has one of the more difficult frame sheets to make.  If you have a different pack, with a more linear and regular compartment for the frame sheet, then it might be wise to begin there.

Please ask if you have any questions.