I just received this model also ... something to bang around while building "real" models. The wingspan is less than advertised, and is indeed flat-bottomed. Bizarre that NP's own rep answered the question incorrectly.
The "Explorer" comes with no fuel-proofing in the engine compartment (raw exposed plywood) - I primed and painted with Lusterkote. The joint where the wing leading edge meets the fuselage also has exposed plywood. I fabricated a balsa fairing and glued it to the wing bottom to close this gap (see photo). Covering match is Monokote Sapphire Blue.
I also reinforced the wing joint with 9 oz. fiberglass, and replaced the under-sized steel wingbolts with 1/4-20 nylon and new blind nuts.
When I e-mailed NP about the correct iron temp to get rid of the hundred or so bubbles in the covering film, I was told "this problem had never appeared before" so they didn't know.
Hoo boy!

PJ