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Generally all you need is a diode (1n914) and clamp resister, 10 K, to the 3.3 Volt bus. This will isolate the higher rs232 voltage from the Mayfly. This is not totally spec (the diode drop gets added to your logic levels) but should work.
Shannon can probably add to this in case he has tried the cheap and dirty solution and it doesn’t work. If not try it since these parts are less than a dollar or floating around in your electronic scrap box. Or Radio Shack if you can find a store that isn’t selling exclusively cell phones!