This part is really not available anymore. I think you can repair if you can find the "fins"? Look on the bottom of the cab for a circular bit of metal with two fins pointing up. The weld breaks and it falls. If you can't find it, I bet you could fabricate one yourself. A tack weld would be close to factory. Maybe a rivet somehow?

Here's a crypt-keeper thread on the topic:
https://pinside.com/pinball/forum/topic/77-bally-saucer-problem#post-22940

Here's the parts guide from Star trek:
https://docs.google.com/viewer?a=v&pid=sites&srcid=ZGVmYXVsdGRvbWFpbnxwaW5iYWxsZmF6fGd4OjQ0MmVmMjRiYzA2NTE4NmQ

This is a photo of the full assembly
Saucer (resized).JPG

faz

A simple strip of metal bent into the shape of a "U" from underneath or an "H" shape inserted from the top would replicate. The saucer does not need a smooth bottom.

The foam is a great idea... just a triangle wedge to send the ball to the switch... nice!
faz