One of the most common and well-known eyepatches for countryballs is that of Serbiaball. In some Polandball communities, it is obligatory that Serbia has one. It is not obligatory in all of Polandball, however, for any countryball that could use an eyepatch to use one.
Using an emblem in a countryball's canton or the center of a flag as an eyepatch is uncommon and not typically recommended by communities. Eyepatches are most common for flags that contain their emblems in the hoist of the flag.
However, this is pretty not used so much and is mainly used for Serbiaball, Austria-Hungaryball (double eyepatch), Spainball, Portugalball Sloveniaball, Slovakiaball, and Skopjeball.