I made my own for the FrankenDuster. I bought 3x3 .120 wall tubing. I took cardboard and made templates of the floorpan, then traced the outlines on the tubing. They fit against the floor like the US CarTool units for a FRACTION of the cost. They are welded to the torsion bar crossmember, the floor and the rear rails. I drilled 3 holes in the bottom, 1/2" in size to make sure any water that may get in has a way out. They were epoxy primed then finished with seam sealer and undercoating. I wish I had pictures to show. They look like Plymouth built the car that way.

On a related note: Who would agree that the TALLER that the connectors are, the more rigidity they add? It seems in theory that 1x3 or 2 round tubes side by side are likely NOT as strong. This isn't meant to derail the OPs "Cheap and easy" requests, just an alternative question.....