Depends if you are making out of an oversized block or plate that is already at the correct thickness.

If using plate I would use a another piece of plate underneath as a fixture, clamp at both ends then....

Mill slot and drill holes(making sure to drill into plate below,
Interpolate two large bores with a slot drill and chamfer
Use a T shaped clamp in both bores to hold while milling the profile.

Then put a couple of pins or drills in the holes and use them to align the part when you flip it over.
Clamp with T clamps again.

Counterbore and mill the two slots that break into bores.