Yeah DS was 7 bang on and she was predicted to be 8,4 I think I guess she was breathing in or the sonography knew I would have bottled out of a water birth.
It is pretty much exactly as you said they are a long thin pocket with a little fold over lip at the top similar to a pillow case
You can make them out if several thing I did mine out if micro fleece but minkee would be super cute too.
If you google bum genius insert sock it should come up. I also use it to use prefolds in other wraps such as te new WN wrap ect. But the wraps do have to be a little bit more fitted