I have seen a carpenter use a home-built arangement with great succes.
In his case, he made three 'sleds' that ran the length of the bed. On each sled he had boxes of parts, etc. To access something, he simply pulled the appropriate sled out until he could reach the box he wanted.
and what happens when a wirenut falls out of somewhere into the track I saw one electrician who had this setup in the back of a full size chevy truck. Not sure if I liked it or not. Sounds great until you consider that stuff shifts around in the bed while you drive, and, as with any neatness scheme- it always starts out with good intentions, but inevitably inside 2 months you are back to chaos. Twice a year I dump my bags out and organize my tool boxes. In between I just ride the wave from organization to chaos till the next time.
I built my own when I couldn't find one the right size for my needs. I love it. Saves my knees from crawling in and out to get stuff. I made it out of channel iron and shoulder bearings. It is 90" long and extends 60" beyond the back of the truck when rolled all the way out. It can support 400+ lbs while extended and locks when retracted and at 3 other positions.
One of my men has one in his F250 and it works good for him...only because he refuses to buy a van. If you are going to use a pick-up for work, this is a helpful tool. It allows you to get to things way in the back without crawling into the truck. I helped him build some short wood partitions to keep thing from shifting.
The pouring rain situation doesn't really matter. No matter what you use for a work vehicle you are going to get wet going to and from. I pull the slide out, get whatever, and roll it back in if its raining. The rear door shelters a good 2' over the tailgate. The doors on my side compartments lift up and shelter the interior from rain unless its blowing sideways. I've worked all day in the rain more this past year than any other I can remember. We're about 10" ahead of the norm for the year. My bedslide didn't cause any new concerns.