I always use a tether something like my current one...
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The Mariner manual says the tether should either be so short the outboard can't rise off the transom or long enough that it can come off and stop due to the kill cord then hang below for recovery.
Their worry at anything in between is that the outboard comes off at speed, keeps running, and powers into the boat.
I've gone for the long option as I thought the very short rope could still allow the outboard to skew round with the prop cutting through a tube end.
Suitable bolt/s with large wing nut so no spanners needed sounds worth looking into.