I did, and it was quite simple, I used 1x1 square with a set of stationary wheels on the front and swivels on the rear. the base was just slightly narrower that my inside track width, so around 24" and the length was around 40", my goal was to build the 4 uprights so that when the bottom kart was "dropped down" onto it, the front nerf bar rails were resting against the bars and the rear tire were just inside the rear uprights, basically trapping the bottom kart in the stand. take a look at Rebel Fabrications stand, his are similar to what we built and he has good pictures.
-Matt