Making ovecab bed more comfortable on Winnebago navion?

We just picked up our new to us navion last night. We plan on doing a big road trip and sleeping in the over cab bed. The bed is pretty thin, I wanted to add a mattress topper. Is there a specific one that is popular here? I was just going to order one off amazon and cut it to size of bed be.. But I thought I'd check in here first and see what other owners have done

Here is a link to a pic of our navion.

https://goo.gl/photos/CeNu8qnxtZzH6iXF6