Here you go Michael, my home made adapter I made a few years ago. I cut up a stock Vanagon jack and welded the post onto some 4X3 1/8″ wall steel tube. The hole to secure the adapter to the jack, using a 1/2″ SS bolt and Nylock nut, was located to suit my jack. Nominal location given on sketch.
I could have welded about one more inch of the Vanagon jack onto the box section, looks like I only did 3″. As is it works fine, keeps the jack body clear of the Vanagon body. Note: as in all the pics on this blog, keep clicking on them to get the highest resolution. Most pics are 800 X 600 pixels, the jack adapter pics are 1000 X 750 pixels.
Addendum: you can buy commercially made versions, Go Westy sells one very much like the one I made, Gary Lee takes a different approach, see here.
Update: after taking measurements and pics, I realised it might look better painted. I gave it a shot of rattle can bed liner.