Simple Case for TubeHobby NCV2.1 Nixie Clock

This is a simple but elegant case for the [TubeHobby NCV2.1-14](http://tubehobby.com/show_det.php?det=42) Nixie Tube Clock.

There are a couple of advantages of this case as compared to [the one I made previously](https://www.thingiverse.com/thing:1347781):

* It is easier/simpler to fit the clock into the case, due to a new design that uses side rails to slot the circuit boards into for a perfect fit
* The overall case design is sleeker, since the top is not a separate piece - it looks lik