Set the 36-inch by 12-inch unfinished wood wall cabinet in place as the front cabinet bed base.
Set two of the four 30-inch by 12-inch unfinished wood wall cabinets flush against the back side of the front cabinet on each side to create the outer-facing side cabinets. The result is a U-shaped configuration of cabinets as a base for the captain's bed.
Run a thin, curvy bead of heavy duty liquid construction adhesive along the outer perimeter of the cabinets. Then place the 1/2-inch pre-cut plywood on top of the glued cabinets and center the plywood on the cabinets as the mattress platform. Let the glue dry.
Run a curvy bead of heavy-duty liquid construction adhesive along the wide side of a 1-by-2 piece of lumber in a 75-inch length. Then center and attach the piece as trim along one long side of the 1/2-inch plywood. Secure the trim piece in place with a hammer and four evenly-spaced 2-inch finish nails and let the glue dry. Repeat this step to trim the other long side of the plywood.
Run another curvy bead of heavy-duty liquid construction along the wide side of the 1-by-2 piece of lumber in a 37-1/2-inch length. Then center and attach the piece as trim along the front side of the plywood. Secure the trim piece in place with a hammer and four evenly spaced 2-inch finish nails and let the glue dry.
Stain or paint to complete the project, if desired.