I tested the surfaces, and verified they were moving in the right directions. The elevator / aileron mixing seems to work well. This model should have plenty of control authority!
After using a dremel to core out space for the ESC, receiver, and various hook-up wires it was time to verify the CG is still ahead of the neutral point. I tried balancing the wing using a 2200 3s, a 1050 3s, and a 2150 4s. All appear to work assuming that the battery has to fit within the boundaries of the wing. Note the batteries are all pictured in approximately the location required to have the CG just ahead of the neutral point (NP).
I'm rethinking my initial plan of not adding any spars or reinforcement except for external packing tape. After I core out enough material for the various battery options there isn't going to be much left at the wing root. I haven't yet decided if I'll add simple carbon spars, or else cut some plywood spars to join the two wing cores across the battery cavity.