I am just going to try to find something that works to 55 degrees and is as light as possible. Here is what I have found, or suggestions made in prior posts. Any other suggestions welcome, although I am trying to get out of this without spending more than around 100-150 dollars (hence leaving off some of the more expensive 1 lb bags):
Silk liner ~ 8oz - Doubt this will go past 60-65 without some clothing support, which you probably wouldnt have on a summer outing. However, it being so light, you could probably bring a jacket and do OK with weight/comfort ratio.
Poncho Liner ~ 18oz - I suspect this would go down to 60.
Lafuma Warm n Light 600 - 23 oz - 45 Degree bag, based on older threads the rating is pretty accurate
Marmot Pounder - 18 oz - EN rating of 54 which seems to be what others have been comfortable to in this bag
Lafuma x600 - 20 oz - Comfortble to roughly 50 degrees
Some other form of liner example: Integral Designs
Brushed Nylon Liner - although I have NO IDEA what comfort level this would provide.