Image of bed's metal top surface without the plastic runner

Good afternoon - I am looking for an image or drawing of the top side surfaces of the bed without the 5.5' plastic covers.

I am attempting to affix a communications antenna mount on top of the plastic by drilling through the plastic into the metal surface under the plastic and do not want to locate the screw holes over an area where there is no metal or perhaps some other object to avoid.

Please see redlined image showing where I'd prefer to attach the mount.

Thanks