Gluing strips to the overhead that you then screw the new bulkhead to may be feasible. The physical joint may work as the boat moves, however,and end up being less than satisfactory with pulled screws and little gaps of broken glue lines
and/or wood fibers that open and close as you sail along. That is why the fillet approach with fiberglass tabbing is used instead. If you're creating your own "custom" laminate, please don't use crummy luan veneer which will look like crummy luan veneer no matter how many layers of varnish
you try to hide it with. Nice mahogany, teak, ash, cherry and other hardwood veneers are available in 4'x8' sheets if you want them. 3/8” ply with decent looking veneers already glued at the factory is also available, though it isn’t cheap. Make a template with cardboard (or luan!) first. You can also simply paint
the new bulkhead white – like Concordia and Hereschoff did. It helps to make the space look bigger than dark wood.
Though a single piece bulkhead would perhaps be stronger, strength obviously isn't needed here - the existing bulkhead doesn't go up to the top of the cabin anyway. Since it appears that there's a seat on the aft side, the simplest route might be to rest the bottom edge of your sheet of ply in the corner at the top of the seat where it currently meets the bulkhead, tilt it upright, and scribe it to fit the overhead. (leaving room for the fillet and tabbing) The seat cushion will make it look like the new bulkhead is the only one. On the other side, where the half bulkhead ends and the new one goes up beyond it, you can perhaps use the top of the old bulkhead to help support a shelfput a shelf (or two). For reading, I would suggest the Gougeon Brothers on Yacht Construction. Hope it cools off for you soon.