Disassemble the bed frame into its component parts, saving all of the screws and attachment hardware for later in the project.
Position the headboard on the ground and set the footboard on top of it so that the longest sides are parallel. Adjust the positioning of the footboard so that the top of the headboard is two inches higher than the top of the footboard. Draw a line along the footboard where it is even with the bottom of the headboard.
Use a saw to cut the footboard along the marked line. Sand the cut edges with medium-grit sandpaper.
Cut off two 4-inch sections of the bed frame side boards. Stand the headboard and footboard upright and have an assistant hold them in place. Position the side boards between them. Position the original attachment brackets between the side boards and the headboard and footboard. Attach them in place with the original screws.
Set the dumbbells on top of the headboard and footboard, with the handles spanning the boards. Mark the desired locations for notches for each dumbbell. Remove the dumbbells and notch out the headboard and footboard at the marked locations so the weights will fit in place and not move around.
Create additional racks as desired and attach them together with wood screws.