We use a double thermarest pad that's about two inches thick that we bought some years ago- it looks like you'd have to buy two singles if you wanted to go that route now since I don't find the doubles on their website anymore. The one we have is probably similar to what's currently called the basecamp model.
It's a huge improvement. Deflated and folded in half crosswise, we can still shut the roof over the thermarest with our down bags laid out on top. It's very comfortable, and they don't absorb moisture like the original foam mattresses. Highly recommend.