It’s easier if you push the top empty area part of the plastic with your finger, and stick the spudger about 3/4” (2 cm) away from the back of the xbox and start prying. You have to do this because that is the only spot where there are no clips in the way. If you do any less or more distance, the clips are in the way, and you’ll never be able to pry open the Xbox because the tool can’t get in.

The T9 fit but was a tad big, for me the T8 worked fine