It may sound obvious but a simple solution to achieving a reasonable aerofoil section is to simply add Depron ribs. As you no longer need the stiffness of the lower skin to maintain the shape both skins can be thin.

It obviously is a bit more complex but the principle remains the same. Bottom skin, add ribs (and spar) then apply the top skin.
In this 40" plank wing built in 3mm Depron there wasn't even a spar and it relied for its strength on the skin alone.

The technique can be taken to extreme as in this 78" span high aspect ratio glider wing using just 2mm Depron but it did have a proper spar.

But simple to build it was not!