Thanks, found it! It's a "mortise chisel and drill": a much more practical solution that I can buy!
While the Reuleaux concept is mathematically cute, it isn't very practical and is only approximate. And the Reuleaux bit requires a special chuck to ensure that the square is "upright" (not tipped at some arbitrary angle): http://upper.us.edu/faculty/smith/reuleaux.htm for details.
There's a joke in this somewhere about mathematicians and carpenters.
http://www.machinemart.co.uk/images/library/range/large/0104...
The small remaining bits at the corners were pushed out by the pointed segments by feeding the bits of leftover wood into the rotating bit.
Worked wonderfully, I wonder during which move i managed to lose it, I always thought it to be a pretty ingenious device.
Mathematically not as clever as the one in the article but definitely simpler and it did the job.