...so I used Fusion 360 to draw out then build a support frame. Here is the design (upside down).
Cut the pieces to length, a few short tack welds later....
I was not about to fully weld it out, as I knew it would warp since I had no way to constrain it in the z-axis, so mostly just a few hot tacks, 'bout 3/8"- ½" long here and there.
Much better to support the weight. I'm not about to set a big-block engine on it either, just something to better support tools/random stuff on the racks.