I have an OR Aurora which I have used a couple of times with a sleeping bag. You can stake down the corners to prevent movement. Its a large volume waterproof polyester bag which I like but this version doesn't have the hoop to keep the fabric off the face although I think it has a tie off so that you can suspend it from a tree branch. I haven't used it in a while. You don't need a separate tarp in addition to the bivy.

Mylar blankets are not blankets and they are certainly not bivies. They work best if not in contact with your skin and used as a heat reflector. I agree with Mors Kochanski and Sweden and recommend these things be taken off the market unless they are not used as "blankets".

I tried the reflective semi-breathable bivy bags from SOL - not one is worth a freaking penny! They are OK for the first 15 minutes to help reflect heat back but then they act like plastic garbage bags and you are soon bathed in your own sweat. Then you are cold and clammy wet - just take these stupid things off the market. 2ndly they are way too narrow at about 31" in width which means you can't use them with any other insulation. Use a contractor garbage bag instead. They are cheap enough that you can use them as a browse insulation bag, as a poncho, or 2 of them together as an emergency tarp.

The newest style I have is the aluminized Tyvek style. It reflects heat back and is able to breathe decently. In fact, I used it this weekend with the temps getting down to -3°C. I was trying out a new hot tent and stove. Hot tents will have condensation in them once the wood stove goes out [then it rains on the bivy] and 2ndly I wanted to protect the sleeping bag from any embers that might fly off the stove. It is large enough (35"x84") to fit my oversized sleeping bag in and adds a few degrees of warmth by itself. I used it last year with just the clothes on my back and slept in 5°C temps with a ground pad. 3rdly, it has a bunch of tie outs and the zipper goes all the way around so I can open it flat and use it as a reflective waterproof tarp as well. This is the version I recommend - 2GoSystems Trifecta