I'd seen a couple of suggestions like this when I searched fhe forums for "floor jack", and read damned near all of the 108 hits (phew!).
I contemplated fabricating something like you suggested, Ray, but I got really lazy when I saw this ad. Also, a neat feature of the Eastwood adapter is the fact that it is designed as a replacement for a floor jack saddle, complete with a bushing to allow it to replace saddles on jacks with 5/8" or 1 1/8" holes. No holding a home-made adapter in place while you position and raise the jack. On the other hand, the protrusion to fit the saddle mounting hole would be in the way if you had some other kind of jack, or a floor jack that this adapter wouldn't fit, and a home built unit like yours would certainly be the way to go.
99.5 2.8 Avant QMS