Hi Ken, Hi All,
This is what the Control-Line Stunt guys have been doing for eons. Typically they core out three sections, but if you are doing a plug-in wing you'll want to core oput two sections, leavinga wide foam spar that is maybe 3" wide at the root and 1/2" wide at the tip. First you bore pilot holes with a hot wire. This is kinda tricky. Then you sheet tha foam, using whatever weight or vaccum bag pressure necessary. Then after sheeting and sanding, the cutting bow wire is fed through the pilot hole, and templates are attached to root and tip. I have even cored just the section of the leading edge outboard of the retract mount by pre-cutting my template hole into the outer retract mounting false rib.

You might try going to the Stunt forums and asking for details. Try www.stunthangar.com and www.clstunt.com and look in the building technique sub-forum.
best of luck, Dean Pappas