As Woodsy says above. Also use a heavier joiner than what comes. Make sure it is not light ply but regular birch ply. You coulddo what I did once with an Eagle II ARF. Add a joiner behind the spar. Cut the covering a bit. Open up a bit of sheeting if required and if you can glue another joiner in behind and against the spar, good. Then do another in front. Not the prettiest, but if you do it under the wing and cover it, it'll look OK.

When it's all joined glass the centre top and bottom and then finish the covering.