I just downloaded the instructions for the SU-26 from AKM's web site. I do not see any reference to covering, or sheeting, the wing with ply or plywood. In the section dealing with building the wing, steps 12 - 16 discuss gluing the leading edge and center section 3/32" balsa sheeting. 17 says to install the 3/32" cap strips followed by 20 - 27 instructing to sheet the bottom of the wing in the same manner as the top. Lastly, 37 says to glue the fiberglass tape around the wing joint after you have joined the wing.

Based on this, I'm afraid I can't help much. You might want to try emailing AKModels to see what they can offer.

Good luck,
John