Its easy to make a common mode choke for this application. I used a ferrite core and wound as many turns as possible through it.
The core to use is the Fair-Rite Toroid Core FT240-31. like the one shown on the right (Yes I am an Amazon Affiliate). Again, making the common mode choke is not critical – other than the ferrite core material needs to be type 31. Using this type should attenuate EMI up to 400 mHz. If you are interested in the specifications of Fair-Rite’s 31 material, you can view the specifications here. Then I put it between my solar panel and the solar charge controller. Next I put a 0.1 uF capacitor from the negative side of the charge controller that was labeled Solar to earth ground.
Here is a photo showing the coil and the capacitor installed on my Windy Nation charge controller.
EMI Filter that stops interference on my off grid solar PV system