Hi Alex,

After trimming with a rectangle, to yield one square tic-tac-toe grid, after MANY boolean unions, and one trim to get an extra side to the inner square,
(and 4 planars to close the 4 open ends,) a solid resulted.
There were some leftover small curved triangular surfaces to delete.

In another limited trial, mutual trim resulted in successful boolean.
The whole grid did not mutual trim, after a couple of minutes.

- Brian

Some other subsets boolean just fine...some do not.

It will be interesting to see Michaels response.