Foam mouldings automatically generate a thicker “skin” on the outside, I assume where the bubbles collapse to become a very dense closed cell structure. You may be able to mimic the effect as @keithski122 suggests but I suspect the best bet would be to make a mould and get some expanding urethane…
It is may be easy to shape and build a good looking foam seat, for it to last and function over time is another kettle of fish entirely.