If cost is not an issue, I you might want to consider silk batting:
https://www.connectingthreads.com/ho...ng/g/HobbsSilk
Silk batting is very soft and, being a natural fiber, it "breathes" and so is very comfortable. It is often used in garment construction (think quilted jacket) because it is thin and has a wonderful drape. It also has a very luxurious feel.
My other choice would be Quilter's Dream cotton, Request weight (their thinnest). Again, being a natural fiber, it will breathe. It also has a very nice, soft drape.
https://www.quiltersdreambatting.com...-dream-cotton/
A third possibility would be Thermore polyester batting, which is very thin. It would not breathe quite as well as silk or cotton, but would still be nice.
https://www.amazon.com/Hobbs-TM90-Ba...dp/B0034C30BM/