Well - we always leave them made up as beds, so from a functional point of view a lump of foam will do.
Aesthetically something a little smarter may be needed. I don't know if we can get a rough match to make up a cover for said lump of foam - maybe someone who is re-covering some similar patterns may have a few usable bits - wait and see.
Long term I must admit I'd prefer to go back to a 410 Triton....