Your bike helmet does not, repeat not, need cat ears.
[beat]
It needs to be a bike helmet which is _designed to accommodate cat ears_. Little plastic cones sticking up, protecting the delicate kitty ears within!
If all goes well, it is not outside the scope of possibility that I could make something like that, in a year or too... but wait, I'd be very nervous trusting your life to anything I made. So maybe your helmet can just get some careful aftermarket modding that doesn't disrupt the protective structure.