I used a 24" wide 2" self inflating foam pad in my BB DL last week and it worked fine. Temps got into the low 40's. The pad did shift a bit but not enough to cause me any problems. The pad is ok for car camping but way too heavy for backpacking.
On the same trip my wife used a WallyWorld blue waffle pad that is 24" wide and she said she stayed warm in her WB DL although she had some condesation issues in the AM. This pad weights 15 oz.