Help - falling in the gap !

We have just bought our first caravan, and for sleeping, use the 2 front berths made up into a double. But we find we are falling into the gap between the long cushions, and also it's a bit bumpy.  We'd like to get a "topper" of some kind, but don't like memory foam, and duvet-type toppers just aren't firm enough.  Thought about a double camping mat, but can't find one which would roll up to go under the berth.   Anyone have any suggestions ?