I found most automatic fish feeders hook onto the end of a tank and/or just didn't seem to fit right for your inexpensive/standard fish tank hood most people get from the pet store.  I made this adapter to fit into the little opening.  It has four holes for the clips in order to secure the adapter to the hood.  The funnel helps to keep fish food going where it needs to go.

The clips are designed to take M3x5x4 brass inserts and 3mm screws.

Main file does have a flange in some models, this flange means it requires support.  There is another file “No_Support” that does not have a flange.

The no flange can be adapted to almost any plastic hood.  Simply cut 30mm dia hole for food, and drill four holes for the screws…or mount with double sided foam tape, etc.