For a light slow flying model the thin shape of the old WW1 airfoils is actually not too bad an option at all. A buddy of mine built a 36 inch SPAD XIII that used the scale airfoil and it performs well with a decently gentle stall.

If you opt for a fully functional scale rigging with the landing and flying wires being functional then the scale airfoil or a flat bottomed with scale upper will do fine. The key here is that by using functional rigging the small section height of the spars will be OK because the rigging ties the two spars together. In the end the upper and lower wings then form a strong structure much like the truss form of a crane boom or climbing crane tower.