Reply from Neville concerning that plywood insert and the excess space. So, looks like I'm back to square one with bonding in a proper plywood insert. I just happened to have some nice aircraft grade 3/16" plywood in a strip a bit over 2" wide. Ripped it down to 2" so that it fit in the rebate areas. Then cut four pieces a bit over 2" long to be bonded in.
See the description for the correspondence with Neville...