Purchase a plastic bin and lid larger than the physical dimensions of your mixing board. As polyfoam will be fitted into this bin to act as a cushion between your device and the plastic shell, ensure that there is room available on all sides of the bin to mount polyfoam while still creating a slot for your mixer.
Cut a segment of polyfoam matching the dimensions of the base of the plastic bin and glue it into the shell. Depending on how much space you have available within your bin, the foam should be anywhere from .5 to .75 inches thick.
Cut out additional segments of polyfoam and glue them to the sides of the bin, ensuring that you are saving enough room to place the mixer inside the protective cushioning. Glue these foam segments to the bin.
Place the mixer inside the bin to ensure that it fits properly. The foam should securely cushion the mixer without having to force the device into the case. If alternations to the foam need to be made, cut off excess portions.
Glue a thin piece of polyfoam to the underside of the bin's lid. Place the mixer inside the bin and secure the lid to the top to ensure that the bin will close properly.
Remove the mixer from the bin and mount the case handle on the side of the bin using the screws and screwdriver. Close the bin and test the handle before placing the mixer back inside.
Close the case and secure it tightly with the bungee cord.