That's the conventional wisdom yes. Monocoque works, but due to the risk of damage in ground handling / incidents - it becomes a poor (and more expensive) choice. The current wing design is an XPS skin that varies in thickness with XPS spar/ribs inside. But the construction man-hours are much higher than we'd like.Stay way from eps in any load bearing application.. One of the easiest to make wings looks like lak-16 - sandwitch construction without ribs or normal spar. Just skin is enought on such a low speed wing.
In my design there's no option to run it as a full monolithic block, but there are areas where it could still be used molded as a component of the overall design.