I have the fabric, but haven't sewn one up yet... HyperD 300
http://ripstopbytheroll.com/collections/hyperd-300
Kyle also sells a 420d that would work great for reinforcements if needed.
Thru-Hiker sells Dynema X in 210 and 120, but it's big bucks.
X-pac fabric won't delaminate, but again, around 20+ per yard and more flavors than is worth discussing here.
Personally I think the HyperD is a good blend of cost, function, durability (especially as you work out the design)
You may consider some cheap 1.9 oz ripstop too just to practice with.
After you make the frame connections (which you can do with Grosgrain reinforcements too) the rest of the pack body doesn't have to be super burly just because it's an external frame pack. The back panel, frame connections, and maybe the bottom will take the brunt of it... the rest of the pack could be relatively light fabric and do fine. If you're not bushwhacking or strapping on lots of stuff anything 3oz/yd or better is pretty tough in my opinion.