We ordered the same foam that Fotochop posted above from the same place in 6" thick for our couch/bed. We made our cushions ourselves and just got them into the trailer this weekend. The foam is really stiff and will not bottom out on our plywood seat base. Maybe a little stiffer than household couch cushions (may settle some after use) but we are glad they are not too soft. Much more comfortable than the original coucho cushions.
The way the foam arrived (2 pieces this way each):
Cut to fit with houshold serrated butcher knife (hey, it worked):