Outboard engine steering lock?

[Adrian NAMT]

I have a new Mercury 5 hp which has a screw to tighten the tension on the tiller when steering.
But, I don't want to steer the boat with the outboard. I want to lock the prop in position and steer with the boat's tiller.
Because the tension screw does not lock the position of the prop, when I steer using the boat's tiller the outboard gradually turns until the boat can't be steered effectively.
The manual does not consider this possibility.
Does any one have a suggestion to replace the tension screw to lock the prop in place?
Thanks for all suggestions.

[Lazy1]

I used a couple quick pins and some paracord this year because I like using the motor to steer in a tight marina but the tller anywhere else.