The water bottle idea I did consider and actually took an old rubber hot water bottle/bladder with me.  I didn't try it as I worried when the water cooled off in a couple of hours that it would be even colder than just the bag.

Yup, my "sleeping pad" is a hodge- podge, but my back and back of my legs didn't feel cold.  It was just the tops of my legs and feet.  Of course, with better padding below, my legs and feet may have been warmer.  BTW, I was on an elevated wood platform with 3/4" plywood top.