Bell-mouth fan inlet

There are some excellent fan ducts for the Printrbot - such as [this one](https://www.thingiverse.com/thing:356001) - that improve considerably on the default duct. However, every one I've used seems to suffer from the same problem, e.g. the fan is trying to push more air through the duct than it really can handle. Part of the problem is that the flow *in* to the fan is in no way helped by the shape of the fan and how it is mounted.

I decided to try a classic bell-mouth inlet and center body;