I used a $25 folding metal sawhorse from
Harbor Freight. It was wider than my chuckbox, but I was able to cut about 6" out of the middle and still have the legs fold into it if they were collapsed all the way down. This provided a very wide and stable footprint, and having each leg individually adjustable made it great for uneven surfaces.
For a long time, I just ran with the two halves of the sawhorse (post-cut) bolted to the bottom of my chuckbox.
The negatives of having it bolted to the bottom of the box were (1) it made the box really heavy to move around and setup, and (2) it prevented the option of setting the chuckbox on a picnic table or bear-box.
So instead I welded the two halves back together and added some end-flanges with butterfly latch catches so I could remove the stand from the box:
With the legs separate, I could get them setup and adjusted, then set the box on top and fasten the latches. With the relatively small top-surface for the box to sit on, attaching the box to the legs was fairly important. You can see that the "work surface" of my chuckbox is cantilevered out from the sawhorse, so if the box isn't fastened down, it tends to want to tip off while I'm slicing, etc.