I have not tried it but it would most likely be a reliable cushion. My enclosures are either 7 ft. or 16 inches for my above ground pens/cage. I have a corrugated steel roof on my above ground pens and I have never had a problem with them 'breaking' their neck.
Sounds like she did flush, if you still have access to her, you could carefully remove the skin around her neck and look for hemorrhaging at the site of the fracture.
You might try egg crate foam on the top, usually available in 3" thickness and variable lengths and widths.