I cut a disc of open cell (very important) foam, about 15mm thick which fitted snug,y inside the cap. This worked very well, catching any oil and preventing it being ejected. I cleaned it in petrol every oil change.
Although the sponge was retained by the springs in the cap, there was a perforated plate in the rocker cover that would have prevented it from entering the cover had it come adrift.
B….
Bernie