Chris, you've got lots of option depending on what kind of performance and weight you want, and how much you care to spend. If you're new to this sort of thing, I'd recommend using something that isn't 20+ dollars a yard. You'll almost certainly want to make a new version with improvements in six months, and will have a better sense of what you want.
That said, 1.4 oz cuben would be the most equivalent fabric. Pricey, and I have misgivings about how well cuben would hold stitching in this design.
The 140D gridstop sold by Thru-hiker is a fine fabric, which is impressively durable for the weight. Very expensive for what it is (a PU coated fabric).
The lighter Dimension Polyant laminates are very waterproof and great to work with. VX07 or TX07 would both work. The former is a bit heavier and more durable, the later lighter and crinkly load at first but also semi-translucent (which I like, it makes finding things in the pack easier). TX07 has a spinnaker-esque feel to it. You can get these from DIY Gear Supply.
Something like a simple 200D oxford nylon would work fine for this pack, and be much less expensive than any of the above options.