Summary

The water tank caps are extremely difficult to remove after the tank is emptied on the Levoit humidifiers because there is nothing to really grab onto. This seems to be independent of how hard the cap is tightened when the tank is filled.
I designed a key that fits over the humidifier tank's cap (works on the 8 tooth and 24 tooth caps). I've also attached the model of the humidifier tank cap just in case you want to modify the design. The key has an outer diameter of 135 mm so it would fit on my creator pro's bed without any problems. This key has made it easy to open up the caps on both of my humidifiers and I highly recommend anyone with these humidifiers make this key. Note: This key should only be used to open the water tank's cap, do not tighten the water tank with this key as the cap should only be hand tight to get a good seal.

Print Settings

Printer Brand:

FlashForge

                                            Printer:                      
Creator Pro

Rafts:

Doesn't Matter

                                            Supports:                      
No

Resolution:

0.2

                                            Infill:                      
20

Filament:

N/A PLA or ABS

                N/A                    
                        Notes:                     
I recommend printing the key at 20% infill and with the top of the key facing down so you won't need any supports.