What are the best practices for designing snap-fit features to ensure ease of assembly?

The geometry of the slot is easier in some ways. It needs to be wider and deeper than the hook. Also, include a small lead-in or radius at the entry.

 You’ll need to find a sweet-spot where the snap is secure but not too difficult to open. Start with CAD simulation if you can. Start with a hook feature that’s a bit too big in an early prototype, and file the hook until it is a bit weak. Remember, make the hooks a little too SMALL in the tooling – increasing them is metal off and cheap.

Great work – one detail though, you could give the snap a little room to move where its tip looks like it hits the lowercase.