Hi Tom, I've attached a tuned up version here - I did an "untrim" on your main body to restore it to its original full shape, then deleted the slightly off round corner in the above screenshot and put in a curve blend in there instead and then extruded a new cutting block from that, then did the boolean.

With this attached version you should now have "all systems go" for doing any fillet radius along that spot.

That particular type of thing with things being only just a 5 degrees or so off from smooth tends to make filleting pretty cranky.

- Michael