A friend of mine made his own switch for this purpose a few years ago. I think he found one with enough wire ports from an electronics supply shop. It worked but the problem is that the switch can introduce noise and he had to do various things to improve the shielding. Also you shouldn't switch transducers with the sounder operating.
I am sure if you ask Airmar or Furuno, they would tell you not to use a switch. If you have good access to the back of your FVC588, just plug in the one you want as you said.