Hi Eric,
Could you say more about where you’re going and for how long?
Here’s what works for a dog and me in the rainforest and tundra for one or two nights— so, worst case scenarios for rain and mosquitos:
8×10 flat tarp, shown below,
and in summer, a floorless bug tent— half the weight and twice the room of a $$$ Mountain Laurel Designs or Zpacks bivvy.
As mentioned above, by the time you add a bivvy sack to a tarp, you are getting heavy and expensive enough that maybe tent would make more sense.
But, everything will get wet anyway, and given enough time, it will dry, too, so just get the tarp. With a large tarp, you don’t need the bivvy’s protection, and have more room to move, or accommodate another person, or a fire. If your tarp collapses, then it could be imagined to turn into a bivvy sack anyway. And you can use your tarp car camping with your tent.
Finally, having had all sorts of shaped tarps, I think you will be happier with a flat tarp since it will be more versatile. You can set it up in half-pyramid style easily, for example, whereas a shaped tarp doesn’t work well for that— the catenary cut adds extra fabric that droops and flaps in the wind.
I also think, once you’ve used a cheap blue tarp for a bit and decided to buy a nice one, you should wait for a used cuben tarp to be offered on Gear Swap. They are much nicer to deal with when wet than a silnylon one, and since you wisely skipped the bivvy sack, you won’t be spending much more money.
You could also buy 9′ of silnylon, hem the edges, and sew on grosgrain loops. That will be around $35 and suitable for one person.
