I'm sure the 66" pad would give a proper fit in the bivy. However, that is still the same concept, though possibly switching from feet to head, as with using a half or 3/4 size pad.

If I'm car camping, I can either use the mummy pad with the bivy or use the long rectangular BA IAC in my Eos1 or Seedhouse 2. No need to worry about weight there. But, for the other times, I'll likely be bikepacking and taking only minimal, yet quite adequate, clothing. If this is going under my head, I don't want to make a pillow out of a salty riding jersey and just-too-nasty riding shorts with chamois!

From the first night I used the bivy over the Winter, I started sleeping without a pillow. (I used to take a full-sized bed pillow and fill the top of my Aether 60 when backpacking). But just like taking to the bivy concept, my body has been loving this flat board/pillowless setup. Just give me my OR Novo Watch Cap to keep the noggin warm thru the night (I LUV that 0.8oz thing so much I bought a spare in case I ever wear it out). Hey, Summer in Vegas, means heading to 7-10,000' and much cooler weather for the weekends ...And I'm off for the Bryce/Red Canyon area for a 4-day July 4th weekend!