I used a BA Encampment and insulated air core pad for a couple years before going to an UQ. It work ok. The pad was a bit small width wise but very doable. I just let the pad deflate about half and it conformed to my hammock.
You can also sew 4 small tabs on your bag and make it into an UQ. I just did this out of curiosity and was surprised at how well it worked. Here's a picture of it from last night. I didn't add anything to the sleeve as it was thick enough. My feet went into the pocket it made on the left side of the picture- I have a WBBB.