Most planes for RC use a ply doubler at the front from firewall to a little past the wing. 1/32 is normal on a 40 size model. This is then covered by the balsa fuselage sides or longerons and stringers. The wing seating often has a trebler of perhaps 3/16 balsa.
You should copy the methods used in successful models of the same type and size. Study a few of the free pull out plans in most British RC magazines. They should show the optimum construction - just strong enough for any flying stresses but light enough to fly well.
Also read books such as Gordon Whitehead’s Radio Control Scale Aircraft Models for Everyday Flying published by RM books Guilford Surrey 1980. Unfortunately this is out of print and selling at ridiculous prices secondhand, but your local library should be able to obtain a copy if you reserve it. This is the best book on the subject for sport scale models.
There is also Designing Model Aircraft by Peter Miller published by Traplet price £7.95 which is a current book www.traplet.com

Most clubs have someone who can cut foam wings, -ask!