We once satisfied a race safety committee for an overnight off-season race with an "Emergency rudder" that consisted of a pre-drilled bunkboard, a couple of Ubolts and the Spinn pole. It was never put to the test (thankfully) and to be honest I doubt it would have lasted long. The forces on the rudders of many boats are considerable (who hasn't leaned into the wheel or tiller as the weather helm loads up in a puff) and to expect some cobbled-together contraption to work long term is a bit naive.
Compounding the problem is that the rudder isn't likely to fail in benign conditions - putting extra strain on the "spare" and making rigging it in the first place problematic as well.
It has been done, people can be quite inventive and ingenious when necessary but for a genuine, usable backup rudder I think it needs to be a dedicated backup, engineered and tested prior to the emergency.