Since you can't support an 8' long piece of ply just at the ends, glass in partial bulkheads at intervals and make one of them at the ply join. You need the added strength anyway. I'd place one partial bulkhead at the corner of the "L", and space two or three evenly the rest of the way. Use epoxy and not polyester for a much better bond. Two layers of biaxial tape on each side of the bulkhead should suffice, after creating a fillet at the bulkhead to hull join.