There are many desiccant holders that fit into the spool centre.  However, I store my spools in vacuum sealed bags, and almost all of my pools are solid.  If the bag leaks and air gets in, it tends to get on the outside of the spool and very little can actually reach the dessicant in the middle.  

I have therefore designed this holder that goes on top of your filament, just below the spool rim.  It prints in a vaze mode, make sure the lid is correctly oriented in your slicer (the wider side must be on the bottom).