I recently decided to upgrade our bumper pull toy hauler for a 5th wheeler. Since the truck is a short bed I wanted to invest in a sliding hitch and came across a Demco Hijacker Autoslide sold locally which was exactly what I was in the market for. In the meantime I also found a 2005 Skyline Freestyle 5th Wheel. At 32 feet and very well kept (original owner) was the perfect package for my use and price.
Turns out that the pinbox on the hitch is only 10 inch wide and the Demco only adjusts down to 12 inches. I called their customer service (which is super, btw) and said that unfortunately they don't offer any sort of adapter plates, but they confirmed that my intent to widen the pinbox was the route many go.
I didn't find any picture online of what people do, so I figured I post a couple of my solution.
It's a 2"x2" angle bar, 1/4" think; MIG welded. I opted for the angle bar to give the side locking plate on the hitch head more surface to work with.
Nothing fancy, but thought I'd share.