You'll need to make sure that the upholstery has some degree of rigidity for structure to keep its shape. The foam itself will not be enough.

E.g. foam with only a thin cotton-like covering? Garbage, will be barely better than a beanbag.

Separated cushions will help rigidity too as they will deform and reduce the load on the main form.

Get the most dense/stiff foam possible to maintain form: you can always soften it with more cushions, you can't do opposite.