Hi Bryon,
I think you saw our headliner at the rendezvous, but I've attached a photo anyway.
I cut 1/4"x 1 1/2" Plywood strips to use as backing boards for our headliner installation. The 1/4" plywood was easy to bend. I attached the strips awthartship to the cabin top with thickened epoxy and used a couple of screws to hold them in place as the epoxy cured.
I used 1/4" closed cell foam between the plywood battens for insulation. The foam is attached to the cabin top with contact cement. I'm not sure the foam insulation was necessary as the cabin top is cored, but I figured it could not hurt to have it.
We prefer to have our interior as light as possible so I installed white 1/8" fiberglass panels which are held in place with 1/4" x 1 1/2" teak battens that are screwed to the backing boards. By making several panels and splitting them at midship it is easy to remove the panels individually when/if I need access to some part of the cabin top.
ps: Your new Mast Beam and Compression Post look Great!
Kevin