Construct a well-crafted platform box using 2-by-12-inch boards. Make cuts with a circular saw. Cut all outermost corners at a 45-degree angle with a miter box saw for a smooth fit. Assemble the corners of the platform with four 1-inch shoulder screws. Measure the base of the platform to fit dimensions a few inches shorter than the mattress size you will install.
Cut cross pieces from 2-by-12-inch boards to fit inside the platform’s perimeter every 18 inches, running from side to side. Insert screws from the perimeter side of the platform into cross pieces so that you can take the platform base apart if you want to move the bed in the future. Paint or stain the platform base.
Build a platform top to cover the bottom base, constructing it to dimensions that are 2 inches wider and 2 inches longer than the mattress. Cut 1/2-inch thick plywood to form the wooden surface that will serve as the platform for the mattress to rest upon. Create this platform area by cutting two thicknesses of plywood. Run the grain of the first layer of plywood perpendicular to the second as one way to ensure seams of the two layers don't align perfectly. You'll need multiple sheets of plywood to do this, but it will give the plywood surface more strength.
Glue the thicknesses together with wood glue and allow them to dry thoroughly for a couple of days. Position the plywood with blocks or heavy objects in place to weight it down so that the sections can adhere smoothly and evenly. Paint or stain the platform to match the base.
Secure the platform to the base's frame by predrilling pilot holes with an electric drill for screws. Attach the platform with 3-inch wood screws that you can remove, if needed, if you need to move the bed in the future. Glue a strip of wooden trim around the platform perimeter at least 2 inches high to keep the mattress in place.