In that venue, how about giving up on the tripod all together and use a sandbag on top of the wall instead?
If you need a tripod for whatever reason, I would advise the following:
- Keep 2 legs short and lay them on top of the wall, possibly with a low angle,
- Hang a pack from the hook (you need a hook of course), make sure the pack touches slightly the ground of the side wall to prevent it from generating additional oscillations.
On top of that, you may want to consider the following too:
- Check for critical sharpness on the spot and increase ISO to reach higher shutter speeds in case you can't get something tack sharp at base ISO,
- Think of using a wider aperture for higher shutter speed and use DoF stacking to regain the DoF you couldn't achieve in one shot.
Cheers,
Bernard