Could be a job for craigyfoam.

Named after my flying buddy, Craig, what he does is grab some soft, spongy, plastic foam & cut a piece to be slightly larger than the cavity which you are trying to fill.

Then mix some epoxy & pour onto the foam (use rubber gloves of course) & when it's soaked through squeeze as much of the epoxy out of the foam as you can. Then pull back your wing sheeting a little & stuff the craigyfoam inside, you may have to push/pull it into the exact position.

The foam will then expand very gently inside the wing filling up the area & dry rock hard.

Finish with the usual patch & paint. - John.