onomatopoeia wrote:
I originally posted this in the Vista section, but received no responses.
I saw your post but don't know the brand they used in our 348.  I won't be to the boat until later in the week (with luck).  But perhaps asking Four Winns as to who supplies them might be a good first step.  I'd imagine anyone making such mattresses would have a way to order one made to custom dimensions.  Usually you end up tracing the existing berth and/or mattress and sending that in as a template.  These folks often have booths at local boat shows.  
We have the inner spring mattress and have found it comfortable.  As has been suggested a viscoelastic mattress (aka Thermopedic) could certainly be cut down to fit.  Personally I don't quite like sleeping on that type but others find them great.