Went to Michaels Aircraft Supply and picked up some foam. Glued two pieces together to make it the right size, then shaped it and wedged it in place. Used the blue epoxy filler to tack it in place.
While that was curing, started in on a few misc tasks that have been stacking up. Countersunk all the rivet holes for the Milspec fasteners.
Then started fitting the lower outboard panel inserts, where the headset and other aux connections will be made. Got the right side drilled and cut to size, along with a backing plate to anchor it to the panel.