Leadfoot,
A couple of options:
1) Perhaps a poncho liner (I would suggest black color, ~ $25).
2) Here's a sleeping bag that I use during spring and early summer here in Texas:
www.sportsmansguide.com/net/cb/cb.aspx?a=129749
It packs into it's own stuff sack measuring 5.5 in. diameter x 11 in. long
Weighs about 30oz. Not sure if that is too heavy for you. It's about $25.

Tango