Ultralight sleeping bag/liner recommendations for bivy sac-ing?

Planning some on route bivvy sac-ing for the upcoming future. Any recommendations for ultralight sleeping bag/liners to supplement a bivy sac, pad, and layers to withstand cold?

hoping it packs up water bottle-ish size. There seems to be pretty good down liners? And also claspable down quilts that pack pretty small?

Wondering if anyone has had experience with this type of setup or any recommendations. Thanks!

Why not just an 850-900 grade down mummy bag?  Depending on the temp rating it could be as small as a Nalgene water bottle?  I think something around a 15F bag is about the size of two water bottles.  Especially with a compression sac to store it in.

I have an enlightened equipment 20F down quilt. It weighs ~19oz and is actually good to 20F. Packs to ~2L. It works super well, exactly as expected. The downside to quilts is the hassle of clipping it together and clipping it to your pad and then shimmying into it from the top, or getting in an open quilt and trying to clip it underneath you. No hood, which is often upsetting. If inside the bivy sac then you cant clip it to the pad, and then the gap that you are supposed to sleep on top of can rotate from underneath you leaving you with a cold spot. Quilts are great if cutting weight is the priority. Quilts are annoying if you are cold and just want to get into your bag and be done. Also, if the plan is to close the quilt anyway, then it's the same as sleeping bag at this point. Unless you wanted to use it for other warmer trips too.

Feathered Friends Vireo is a very good ultra-light solution.  I've used it on bivis with good success.  

https://featheredfriends.com/products/feathered-friends-vireo-ul-down-sleeping-bag

I use a quilt, and with a bivy sack it’ll be even more effective because drafts are less than an issue. I like UGQ quilts, a bit warmer and more durable than enlightened equipment. 

Thanks for all the help! Gunna start scouring eBay and the web for used quilts.

I would just look at getting a pair of down pants if you are really worried about weight. It all depends on how much you want to suffer.