I found that after using my water bladder in my hydration pack that it remained wet inside for an extended period of time.  I tried a few of the inserts that hold the bladder open but it still remained wet after many days.  This print uses a 12v 60mm fan, a voltage converter, and USB-C header to create some active circulation for it to dry quickly. 

• Hydration pack I have from Mubasel Gear. 
• Voltage Step Up from Amazon
• USB-C socket - these are not the exact ones but they all look the same
• USB-A to USB-C cable.  You probably have some these lying around

Print supports not required

To assemble

• Use M3 heatsets for the body of the fan mount then screw the fan and fan guard down.
• If you are using a > 5V fan, tune the step up converter to the fans voltage
• Solder the input side to the USB-C socket and the output side to the fan leads. Remember the wires have to pass through the electronics box USB-C opening before you solder them on both sides
• M3 heatsets for the electronics box, hot glue the voltage converter down, and screws the lid on to the box.
• The USB-C sockets only run on a USB-A to USB-C cable 

The fan guard is from this model

F360 file included for anyone who wants to tweak it for their needs