Make a template in flat wood or ply. use double sided tape to stick it the piece and use a bearing guided flush trim bit.
I like that jig, Tom, thanks for posting.Here is jig that could be modified, although I don't see why a jig would be required to put a radius on a part that's only 3 mm thick (assuming that the 3 ml was a typo). A picture of the part may help with giving suggestions. The same concept would work on a belt sander.