Customizable PLA cooler

**NEW:** see the **PLA Cooling Challenge** http://www.thingiverse.com/thing:627590

PLA layers need to cool before the next layer is printed. If the layer is small (that is, doesn't take too long to print), it may still be warm and soft when the next one is printed which may cause visible deformation and reduces print quality. The problem can be mitigated to some extend by slowing the printing speed or using a material cooling fan but is still significant with very small layers.

This thing