How can we design a wing for cruising at very low speeds?
The first thing you need is a large wingspan $b$. This will help to spread the lift over more air and reduce induced drag, which grows inversely with the square of flight speed $v$:
$$D_i \propto  \frac{1}{v^2} \text{ and } D_i \propto \frac{1}{b^2}$$

Large wings have a high root bending moment, which requires a strong and heavy structure. Next you should add bracing, at least on the lower side of the wing. For that to be effective, you need a high fuselage so the bracing will become efficient. Yes, those wires will add drag, but at low speed this will be a lot less than the added lift-induced drag from lifting a heavier structure.



The single bracing wire can hardly be seen on this picture of the Dedalus human powered aircraft, but it is there and saves a lot of drag at low speed (picture source)

Next, pick the right airfoils. Depending on the local Reynolds number, use thin and highly cambered ones but make sure that the pressure rise on the rear upper surface is slow enough to prevent early separation. One example is the DAE31 outer wing airfoil of the Dedalus aircraft (the inner and mid-span ones are DAE11 and DAE21). At Reynolds numbers of 500,000 to one million, turbulators positioned 2-3% ahead of the separation line will trip the flow and make the airfoil tolerate steeper pressure gradients.

Pick wing chord such that the wing area gives a lift coefficient around 1, details depending on the exact Reynolds number. Lower values go with lower Reynolds numbers.