See details in Radek's original design. It's basically an air duct to direct air from a 120mm CPU fan into the hydration bladder to speed up drying. Can also work without the fan, to keep the bladder open and facilitate draining / drying. The shoes are optional and should be printed in softer material, like TPU.

The model rev 3 has a tapered air inlet to allow more airflow from the fan. I used Tinkercad, which doesn't seem to make it easy to gradually decrease the diameter of a tube. Maybe one day I'll redo it in OnShape if I get a chance.