One of my cats would reach his paw up into the feeder and turn the little turnstile so that it'd dispense food. In order to prevent him from reaching up there, I made this plate that reduces the side of the opening.
There are two slots for zip ties to mount it to the feeder. One goes horizontally to make sure the plate hugs the feeder. There's a little half-tube on the inside to keep it aligned laterally.
Another zip tie holds the plate down vertically since my cat would also push the feeder over. This would cause the food bowl to fall off so I dremel'd a couple slots into the bowl and slotted a zip tie between there and the cover plate to ensure that he can't take the bowl off.
For an attempt at food safety, I coated it in linseed oil and beeswax. There shouldn't be much contact between the plate and the food, but it should help.
Fair warning, I'm not very good at CAD so the model probably sucks.
The author marked this model as their own original creation.