The first problem: due to its shape (tapering angles) I can't accurately measure the size of the triangle.
Solution: make slightly oversized model and see how my shapes fit on top.
If I haven't totally blundered in making the shapes, the angles should match....
and they do.
So do the angles on the brackets.
in the moment of practical inspiration I drew the clay shape on the transparency and copied it 6 times. Position of the holes is important.
And there I found it!
The hole on the bracket needs to be 5 mm further from the center. Therefore the bracket needs to be 5 mm longer on each side, as you can see on the paper model (right).
So, back to the bracket making, measuring, connecting and testing the theory....
and this time it works:
Do you want to see it right way up?