It's Dimension Polyant VX-07, not a hybrid polyester-DCF (formerly cuben fiber) like Hyperlite Mountain Gear and Zpacks use for their packs and accessories. I chose it primarily because the pack was something of an experiment, and I still had most of a yard of VX-07 leftover from a previous pack project. I'm really happy with the pack design, and if/when it wears out, I may have one professionally made with some more durable materials.
In case you want to get technical, VX-07 a 70 denier face fabric laminated to waterproof layer, an X-ply, and a white polyester scrim. Truthfully, it's the lightest I'd go for a pack of this kind--VX-21, which ups the face fabric to 210 denier would be my first choice. It doesn't see a lot of abrasion, so it's holding up fine so far. I also have another chest pack that is made with the same VX-07, and it saw two years of good use before I moved to this version. It's in great shape, other than my poor sewing.
The other change that I'd currently make is to go with the more pliable and durable 210 denier Dyneema X for the front pocket. I already have put a small (easily fixable) hole in it from not being careful and dragging it across some rocks (totally stupid and unnecessary, too).
I have a larger, fly fishing specific day pack design that I may build out this winter, but my Osprey Talon 22 still performs well enough that I may not take the time yet. If I do, it will be based on the same fabric families as this one--a durable Dimension Polyant laminate for the main body with Dyneema X for the pockets and accessories. We'll see what happens.
If you're interested in one, I'd recommend having Chris Zimmer of
Zimmerbuilt put one together for you. His quality will far exceed mine, and if you need more pictures or dimensions (I'd customize them to your fly boxes), send me a PM. Chris is a class act and an ace at the sewing machine. You'll notice that a lot of the inspiration for this pack design came from his work.
If I'm still happy with this design when the current version wears out, he's the one I'd go to for a rebuild.