In an earlier post, I mentioned overcutting the inside corners of the parts so that they fit together better. Below are diagrams that show what this looks like.
You should decide whether it’s more important to have a tight fit or easy assembly.
If you go with “tight fit”, you should design the slots slightly undersized (0.005 or so), and fix it by filing away any excess. Heating the hole section with a torch or heat gun so that it expands will make putting the pieces together easier. You can force the pieces together with a press or even a hammer.
If you want “easy assembly”, you can get the parts to hold together more tightly using retaining compound (Loctite makes a good one) or epoxy.