The concept here is called kerf correction, kerf meaning “how thick the cut line is from the laser”. Since the laser cut has a certain width to it you have to compensate to get a truly snug fit by making the cut shape slightly smaller.
So good news, bad news. First the bad: this doesn’t have just one answer. Exactly how thick the cut line is varies depending on your material and settings. This makes it hard to say for sure how wide your slot needs to be.
The good news: for most 1/8” thick materials 0.006” or 0.152mm is about right so a slot width of 2.348mm is a good guess.
And there’s the tricky part: it’s a guess. The best way to get it exactly right is to test. I like these little guys:
Anyway, there you go. Sorry it’s not a simple answer.