I've actually cut angles slots as exhaust vents in a set of side covers, but it was expensive in the form of using many dremel disks and it was very time consuming, so I never recommend it. The end result looked quite nice though.
You'll find the resulting shape to be angled slots with sharp edges (the leading edge on the inside and trailing edge on the outside). Be careful in the programming, because angle in the cuts for these slots depends on whether the cover is for the left side or right side, and if you get it work flow will suffer drastically, because they will try to scoop air in at the same time centrifugal force is pushing it out.
For the intake, instead of slots parallel to the axle like for the exhaust, I would cut even narrower slots (to keep debris out which can obviously flow in the intake easier than into the exhaust and whatever grit and sand does get in can flow right out of the exhaust). The goal of intake is to allow free flow of air into the motor. That means as much cross sectional area as possible, and to the extent the holes can be shaped like blades that "scoop" air into the motor it is a good shape. Again, the angle is important, because it is opposite on the left and right sides. I would cut many slots running radially from the axle, and stagger the roots of the slots to make room for more of them.
John