You might be able to get the folks at Mountain Laurel Design to make you a Spinn one. I did get one of the last ones they had in stock a couple of months ago. I stripped it down (took off the linelocs and used really lightweight cord for guy outs and ditched the stuff sack it came with for a cuben one) and got the weight to 5 ounces. The coverage is decent but I would not trust it in "Sandy" weather.
You might also get Warbonnet guy to make you one out of Silnylon; can't hurt to ask.
I expect Z-packs is the lightest though. Cuben is nice stuff!