Fixed cup width

[oldandslow]

Definitely get the tool for the job. I proper BB spanner fits around the cup on all sides and gives more purchase.

 

[Punkawallah]

If you can get a nut, bolt, washer and plate arrangement of an appropriate size it’s useful for holding the spanner in place to reduce slippage. I use a nut/bolt/plate arrangement from a side stand.
The nut/plate end goes outside the cup, the bolt/washer end goes inside the cup.