Or possibly take the cover off and add two pieces of plywood or some 2x4 runners to reinforce their strength? The foam is prone to be indented or could feasibly crack. Most have a 300lb load capacity but that is more for things like snow that would be evenly distributed, versus the dead weight or a person.
I stack spas atop spas in my warehouse so I know the spa can take the weight. I'd prefer if you took the cover off and put some runners across the tub and toss some plywood over it if you could.
I hope this helps. Good luck moving forward.