I used the HF die grinder, and a Black & Decker Workmate to hold them. I think it was 100 grit pad. Just start sanding, side to side, concentrating in the middle a little more. Once the ribs appeared in middle, started working towards the outsides,I know some that used a nail file to smooth out afterwards, pretty simple, just take a little bit at a time, not too much pressure when sanding.

As my esteemed colleague Bjorn stated, not sure about the functionality of having them open, but it looks cool and makes for some good discussion Kel