Get some scrap wood from Home Depot. I like to use the pieces with cutouts and varying color, but similar shapes and size. Line them up to the length of your bed.
Use liquid nails to attach. Leave drying overnight.
OPTIONAL: screw two unique pieces to each end for a more complete look.
Screw a piece of plywood into the back for support.
Paint with desired stain.
Let dry and you're done!