half bag

elephants foot is what they used to be called..
Nunatak makes the 40° Arc AT for $213
Western Mountaineering makes the 30° Tamarak for $200

if you want a te-wa top quilt, in a 50" long size, It will be a tapered rectangle (trapezoid) with a lay-flat footbox that is formed with velcro and drawcord.
I can do the 30° temp rating you seek with about 2" loft.
Takes 10 days or so to build.. PM me for cost and details.
here's some info: http://www.te-waunderquilts.blogspot.com/

Feathered Friends also makes one, called the Vireo: http://www.featheredfriends.com/Picasso/Bags/Specialty.html

Here's one you might be interested in. They don't get much of a run at BPL, but I'm pretty sure that the UK members would be able to give some feedback about the company. Mike? Chris? Peter?

http://www.alpkit.com/shop/cart.php?target=product&product_id=16329&category_id=253

The Pipedream 200 is rated to -3C (about 30F) for about US$100. This includes VAT, so when you take VAT off, and add on shipping, you may find it's actually cheaper than that, delivered to the US. (To Australia, it's about US$98)

Weight is 460g, including the sewn on, uninsulated vest, which makes a lot of sense. I'm guessing it is about 60g, if you want to cut it off.

Alpkit are a small UK company, and only produce small batches of gear at a time. I haven’t used one of their bags, but most users seem happy with them. Although not the lightest, they seem to be good value.
Another half-bag is the OMM PA1.
OMM make a lot of nice light gear that is popular with the ultra/mountain marathon running community here in the UK.