Lots of companies make liners and covers. I would think a Pertex or eVent (you would need one that breathes) bivy would add about 10 degrees of comfort, and a good liner would do that or more.
Here are some liner options:
I just bought a Stoic Vamp 15F Down bag, from Backcountry.com, pack weight is 2.9 pounds the reveiws were good and It was on sale for $143. It is still there if your interested. If you dont like it send it back money back - the shipping.
Def still interested Red, Its still pretty chilly and until it gets warm Ill just be doing day trips or longdistance scouting and hiking back and staying in my camper/truck No need to get right on top of them and scare them off. Let me know how you like that one.... Also thanks for the info on the Cats meow bag!
Seeing this today reminded me about this thread on the question of liners and sleeping bags. Boosting temperature ratings, sleeping bag options and it also adds as an emergency bivy. At 8.5 ounces this looks like a possible solution for some of these mentioned on the thread. Heat reflective, waterproof and breathable.
I use a Big Agnes Horsethief (35 degree) bag in California archery season and if it gets cold i use a sleeping bag liner to increase the temp rating 15 degrees. I have not been cold. I found mine used at www.bwear.com for $89.00. Just keep watching their site and they have some great deals.