I discovered today that Thayercraft, a fiberclass supplier, is selling 3.3oz/yard pure Spectra 1000 fabric (roughly equivalent to Dyneema sk65) on their website and on Ebay. That weight seems about right for a pack, and I thought some of the MYOG folks might be interested. It's only $20/yard (60" wide), which is amazingly cheap for pure UHMWPE fabric, but the minimum purchase is a ten-yard roll. I don't know the details of the weave.
I'm not associated with Thayercraft, I just saw the Ebay listing.