I needed to get the exhaust nuts off my new project bike (1984 BMW R65LS) and couldn't find back the cast iron spanner I last used 13+ years ago...

OpenSCAD and my trusty 3D printer to the rescue.

Some Googling showed me enough information to design this, along with a nut to take measurements off.
For the material choice, I watched this: https://www.youtube.com/watch?v=_rU873Zw9kk

I designed it to be printed flat on the bed, for strength.
Since I designed it in 2D and extruded it vertically, the corners are a little sharp, so some hand covering is suggested when using it.

The tips do flex inwards still, but when used in anger, we could NOT break it.

The nuts were stuck on VERY solidly and I wasn't sure the spanner would stand up to being hit with a hammer. I used a brass drift and hammer to shock the threads, whilst a helper was also applying pressure on the spanner. With this technique, both nuts were removed and the spanner is still in perfect condition and the nuts weren't severely damaged.