https://www.google.com/search?q=truck+bed+bike+carrier&client=safari&hl=en-us&prmd=sivn&source=lnms&tbm=isch&sa=X&ved=0ahUKEwijpqmtzqTSAhWogFQKHamgAAYQ_AUICCgC&biw=375&bih=559#imgrc=Hl5dAhc3IJ919M:

I have a bed cover that has a track that won't allow me to easily attach the bike to the top of the bed rail. This allows me to locate the bar anywhere within bed I want.

I've seen versions of this concept homemade by individuals but they are usually connected to the bed of the truck.