Peter, Is this what I think it is, a sit upon? Do you have plans, dimensions, bill of materials, etc?
Larry, ya got me. I don't think I know what a "sit upon" is. Clue me in...
These are sawhorses, I use a couple of them on outfeed and left side of my tablesaw for support. That particular one happens to be the first one I built and I use it to horse sheets onto the saw, either off a stack or leaning against the wall.
I may have a Sketchup model of later iterations, but I doubt it's what most would call complete. I would mostly use it for a jumping off spot, sort of completing the design as I go. If it interests you I'd be happy to send it your way.