If your using o/s grips then you could either cut the end flange off by hand, or turn it down in a lathe. Either way would work as the grubb screw is still visible to be able to clamp it to the inside of the bars

When you put your o/s grips over the top of the extensions you wouldnt be able to tell any difference compared to the end of a normal bar

Job done....Slight bit of a modification to the extensions but it would definitely work.