The idea was sound...

The 2nd attempt on a papier mache cup failed (no pictures of yet another failure!). The hotwire-cut polystyrene balls are simply too uneven in shape which caused big cavities between ball and cup.

Luckily, the woman at Graham Sweet Studios (where I purchased the polystyrene balls from) mentioned that the inner diameter of the 250mm hollow-half spheres might just be the right size. So I tried.

I cut about 2cm off the edge of the 250mm half sphere as that should leave enough of the cup for the aircushion to form and at the same time allow more room for uneveness of the ball:

Here is the result:

The result isn't yet convincing, but the key problem here was again the unevenness of the ball rather than the idea. To circumvent this, molded 200mm balls, which come in two hollow halves, have been ordered. With any luck, they will fit nicely.