It took a couple of iterations, the first case I made was a 1mm wall thickness, and it just felt too flimsy. For V2, I rebuilt it with a 2mm wall thickness, and it works like a charm. The one downside to the design I created is you have to glue it closed which means if you ever want to get the Teensy out, you'll have to destroy the case.
It'd be nice to have something that either clips together or can be closed with a few screws, but that sort of thing is beyond my limited modelling abilities.
-- Edit --
Note that my model is for a Teensy 2, you'd need to rework it for a Teensy 3, since the USB connector on the 3 is a micro vs the mini used on the 2. That and the fact that a 3 is 2/10" longer than a 2, IIRC.