I presume you are saying you want to have a square border around text which
is at 45 degree angle. If this is correct, The only way I can see you doing
this is to add a row above, below and column left and right of cell which are
set very narrow in depth and width accordingly then select borders and
'outside'. This will place a border around the outer cells appear as though
it is only around the cell with the text on 45 degree angle.

Hope this helps!

cheers
Tanya

Format/ Cells/ Alignment.
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David Biddulph