I added quarter- by half-inch pine to the back edge of the thin plywood base to stabilize it and add more surface area to attach the frame to. The plywood itself is salvaged from the crates I built to ship my builds when I moved three years ago :)
After painting the inside frame pieces with two coats of semigloss latex, I cut the two-inch ply lengths to fit and glued them on the base, butting the corners. I used binder clips to hold the frame to the pine strips and a flexible strap corner clamp thingie to hold the frame square while the glue dried. A first coat of paint dries with Albie’s supervision. Tomorrow I’ll tap in tiny nails to secure it all, then fill the holes, sand and paint, sand and paint, sand and paint. And hopefully this weekend I’ll have collected more plastic to call it done.