As mentioned above, the underquilt insulation will compress under your body weight and will not be very insulating.

An inflatable insulated pad sounds like your best bet. The "R" rating should be 5 or above for cold weather. Don't pump it up full; for the best comfort it needs to be a bit squishy. It will be a bit less insulating when squishy, but really comfortable.

With my Exped UL Downmat, in squishy mode, I have never felt the need for a cot, even though I'm in my 80s and getting rather decrepit, with corresponding aches and pains.

For any sleep system, or new components, I do strongly recommend several nights of trial at home before taking it on a trip, while you can return it!