I replaced the aft cabin mattress in our 380 in just a few hours and it was inexpensive and oh so comfortable! We ordered a 6" memory foam mattress online along with a quality 2 blade electric knife. When it arrived we opened it up on the dock and let start expanding while we pulled the old cushions up to draw the pattern on the new mattress.
Now here is the funny part. In our haste we didn't cut the mattress upside down (has an angled area for the hull at the head), so when we got it down we actually had to install the memory foam on the bottom and we sleep on the dense supporting foam. Here is the great news!! Memory foam mattresses are notorious for not breathing and making you sweat. Because ours is upside down, the foam we are on breathes excellent and fantastic to sleep on. Now and then when we leave and strip it down, I just grab the mattress and slide it half out to the engine compartment and leave it to get fresh air under it.
Sweet Dreams