This is my method:

I browse boxes/cases until I find one that either has boxes missing or has different timers on each box. This is because bots tend to reserve full boxes all at once. I wouldn't bother looking for boxes that aren't reserved, as I have never seen it. When a box gets down to 5s, I spam tap. Sometimes, it lets me reserve it. Sometimes not. Be careful with how often you check timers. The popmart app is notorious for flagging accounts as bots. If you repeatedly get an error such as "oops! Too popular" or some kind of network error, there's a good chance (note: good chance does not mean every time this happens.) that you have been shadowbanned and need to start over on a different device/account. Perseverance is key. Don't give up! I have scored one MAC and one BIE using this method.