How to Use Ravioli Maker without Press Insert
Just push down gently with your thumbs; you want an impression that's about 1cm deep.  There is no way to ensure uniformity if you have lost the insert. 

You can just buy a replacement ravioli maker, which will have the plastic plate.  I suppose you could even fabricate a replacement for the missing plate, but anything you make is liable to be more expensive than simply buying one.