Peter,
Just to clarify, I didn't use the Remove Rigid Body Modes feature in the last simulation. The description in F360 documentation is: "At times, it is difficult to constrain a model without limiting or preventing the deformation that would occur naturally. If you choose to leave your model unconstrained, you can select Remove rigid body modes to eliminate rigid-body, or free-body, motion during the solution." I could be misunderstanding it, but that sounds like the situation I'm in. I wouldn't use it for general purposes, because I don't know how to make the sum of forces and moments zero in a complex simulation. But I'm using it right now to model my table by itself with an arbitrary force applied right to the middle of the table, and gravity is turned on. If I am constraining the feet, ideally I would just constrain one of the edges or vertices to slide on the floor, right? Since it is unrealistic for the whole flat bottom of the table legs to remain attached to the floor.
About using PU: you've scared me away from using PU. If I were going to use it, I would probably want to let it cure in a rough mold and then cut it down to the exact shape that I want. The other thing is that I think that it will be more economical to achieve the properties I need for the core by using steel ribs. Getting PS foam wedges to couple will be harder with steel ribs, but I think if I cut the foam and steel accurately, I will be able to use epoxy and fumed silica to fill in any gaps and mate all the surfaces well enough. I have some polyester and vinyl ester resin that is going to go bad and I've been looking for something to use it on, this could maybe be a way for me to use it up. Or I could just do the job properly with some epoxy.
I appreciate your experience and wisdom. Switching to a concrete design would put me back weeks if not a month, which is not really feasible for my business right now. I feel good about the potential of this design: it seems like it will be a great deal for the work area and rigidity. There are just a few things that need to get worked out.