This is a simple case for the Sparkfun Tiny AVR Programmer.
The case is designed as snap-fit enclosure. Just insert your Tiny AVR, USB port first, then click the bottom into place. No glue or screws needed.
The case itself is not super tight (ran out of patience messing with tolerances) so the board rattles a little on the inside. If that bothers you, a little hot glue can solve your problem. The case leaves the prototyping pins and IC socket well exposed for ease of use.
I recommend a minimal support on the Top part to give you a clean opening for the USB port. The lip will be covered by the USB port, so you'll never see any support structure artifacts.
STL's should be properly oriented for printing.
If you need more info about the Tiny AVR, please visit Sparkfun.com