The spanner can be mounted where the mast base is already riveted/bolted and then the control lines are led to fairleads and cleats mounted on the main beam. I don't have one on my boat though and seem to do fine in 15-20kts. If you are not already doing this, you should move the clew/outhaul inward and that will help flatten too and usually results in a little less rotation with the sheet tight.
This is probably what I need to do, attach main sheet further in on the sail to flatten the sail and pull the mast straight. The problem is that I lose tension on the square top and the sail inverts...
