RE: How do I balance this thing?
A couple of 3/16" birch dowels that pass through the wing saddle to hold the rubberbands......or sew the dowels to the front and back of the saddle.
It looks like the CG will be just in front of the spar and I'm guessing you might have to move the engine back......just guessing from appearances.
Congratulations, you've got a new skill that will be valuable to you as long as you choose to build with foam.