How do you fixture a part that sits on top of a table for simulation??

How do you fixture a part that sits on top of a table and is not bolted down to the table?  I want to run a simulation to see what a hollow cube would do if I pressurized the inside volume.  I think I can use elastic support but I've never used it before and I'm not sure how to set it up right.

SolidworksSimulation