Thanks for the reminder. I have now a 14 mm (about 0,55") good quality pad from Fjällräven. If Brandon is correct, it should be good enough for singe digits in F and so it would be usually enough. But with Winter yeti it should fill all my needs. Cutting pad material mini cell or similar to fill full double layer area in BB would mean that the Yeti would not work correctly, so smaller is better in this case I think.
If I was not sick I could test those, there is -29 C outside about (-20 F).